diff --git a/.travis.yml b/.travis.yml
index b75e324..3870849 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,4 +4,4 @@ rvm:
   - 2.4.0
 
 env:
- - "RAILS_VERSION=5.0.2"
+ - "RAILS_VERSION=5.0.3"
diff --git a/Rakefile b/Rakefile
index e48a9ea..269cd35 100644
--- a/Rakefile
+++ b/Rakefile
@@ -4,24 +4,31 @@ require 'rspec/core/rake_task'
 
 RSpec::Core::RakeTask.new(:spec)
 
+task :default => :ci
+
 task :ci => ['generate_test_gem', 'spec'] do
 
 end
 
 task :generate_test_gem => ['engine_cart:setup'] do
-  system("rm -rf .internal_test_gem")
-  gem 'rails'
+  destination = EngineCart.destination
 
-  rails_path = Gem.bin_path('railties', 'rails')
+  system("rm -rf #{destination}")
+
+  if ENV['RAILS_VERSION']
+    gem 'rails', ENV['RAILS_VERSION']
+  else
+    gem 'rails'
+  end
 
   Bundler.with_clean_env do
-    system("#{rails_path} plugin new internal_test_gem")
+    system('bundle exec rails plugin new /tmp/internal_test_gem')
   end
-  system("mv internal_test_gem .internal_test_gem")
+  system("mv /tmp/internal_test_gem #{destination}")
 
-  IO.write(".internal_test_gem/internal_test_gem.gemspec", File.open(".internal_test_gem/internal_test_gem.gemspec") {|f| f.read.gsub(/FIXME/, "DONTCARE")})
-  IO.write(".internal_test_gem/internal_test_gem.gemspec", File.open(".internal_test_gem/internal_test_gem.gemspec") {|f| f.read.gsub(/TODO/, "DONTCARE")})
-  IO.write(".internal_test_gem/internal_test_gem.gemspec", File.open(".internal_test_gem/internal_test_gem.gemspec") {|f| f.read.gsub(/.*homepage.*/, "")})
+  IO.write("#{destination}/internal_test_gem.gemspec", File.open("#{destination}/internal_test_gem.gemspec") {|f| f.read.gsub(/FIXME/, "DONTCARE")})
+  IO.write("#{destination}/internal_test_gem.gemspec", File.open("#{destination}/internal_test_gem.gemspec") {|f| f.read.gsub(/TODO/, "DONTCARE")})
+  IO.write("#{destination}/internal_test_gem.gemspec", File.open("#{destination}/internal_test_gem.gemspec") {|f| f.read.gsub(/.*homepage.*/, "")})
 
   Rake::Task['engine_cart:inject_gemfile_extras'].invoke
   EngineCart.within_test_app do
@@ -48,11 +55,10 @@ task :generate_test_gem => ['engine_cart:setup'] do
       system %Q{echo '\ngem "sprockets", "~> 2.11.0"\n' >> Gemfile}
     end
     Bundler.clean_system "bundle update --quiet"
-    system "echo 'require \"engine_cart/rake_task\"\n' >> Rakefile"
 
+    system "echo 'require \"engine_cart/rake_task\"\n' >> Rakefile"
     system("bundle exec rake engine_cart:prepare")
     Bundler.clean_system "bundle install --quiet"
   end
 end
 
-task :default => :ci
diff --git a/lib/engine_cart.rb b/lib/engine_cart.rb
index d119eaa..911fc4c 100644
--- a/lib/engine_cart.rb
+++ b/lib/engine_cart.rb
@@ -1,13 +1,14 @@
 require 'engine_cart/configuration'
-require "engine_cart/version"
+require 'engine_cart/fingerprint'
+require 'engine_cart/version'
 require 'engine_cart/gemfile_stanza'
 require 'bundler'
 
 module EngineCart
-  require "engine_cart/engine" if defined? Rails
+  require 'engine_cart/engine' if defined? Rails
 
-  def self.load_application! path = nil
-    require File.expand_path("config/environment", path || EngineCart.destination)
+  def self.load_application!(path = nil)
+    require File.expand_path('config/environment', path || EngineCart.destination)
   end
 
   def self.within_test_app
@@ -18,30 +19,9 @@ def self.within_test_app
     end
   end
 
-  def self.fingerprint
-    @fingerprint || (@fingerprint_proc || method(:default_fingerprint)).call
-  end
-
-  def self.fingerprint= fingerprint
-    @fingerprint = fingerprint
-  end
-
-  def self.fingerprint_proc= fingerprint_proc
-    @fingerprint_proc = fingerprint_proc
-  end
-
-  def self.rails_fingerprint_proc extra_files = []
-    lambda do
-      EngineCart.default_fingerprint + (Dir.glob("./db/migrate/*") + Dir.glob("./lib/generators/**/**") + Dir.glob("./spec/test_app_templates/**/**") + extra_files).map {|f| File.mtime(f) }.max.to_s
-    end
-  end
-
-  def self.default_fingerprint
-    EngineCart.env_fingerprint + (Dir.glob("./*.gemspec") + [Bundler.default_gemfile.to_s, Bundler.default_lockfile.to_s]).map {|f| File.mtime(f) }.max.to_s
-  end
-
-  def self.env_fingerprint
-    { 'RUBY_DESCRIPTION' => RUBY_DESCRIPTION, 'BUNDLE_GEMFILE' => Bundler.default_gemfile.to_s }.reject { |k, v| v.nil? || v.empty? }.to_s
+  def self.update?(fingerprint)
+    return true if EngineCart.fingerprint_saved.nil? || fingerprint.nil?
+    EngineCart.fingerprint_saved != fingerprint
   end
 
   def self.configuration(options = {})
diff --git a/lib/engine_cart/configuration.rb b/lib/engine_cart/configuration.rb
index 093136e..694b48e 100644
--- a/lib/engine_cart/configuration.rb
+++ b/lib/engine_cart/configuration.rb
@@ -11,15 +11,15 @@ def initialize(options = {})
     end
 
     def load_configs(config_files)
-      config = default_config
-
-      (default_configuration_paths + config_files.compact).each do |p|
-        path = File.expand_path(p)
-        next unless File.exist? path
-        config.merge!(read_config(path))
+      @config ||= begin
+        cfg = default_config
+        (default_configuration_paths + config_files.compact).each do |p|
+          path = File.expand_path(p)
+          next unless File.exist? path
+          cfg.merge!(read_config(path))
+        end
+        cfg
       end
-
-      config
     end
 
     ##
@@ -79,12 +79,12 @@ def default_config
 
     def read_config(config_file)
       $stdout.puts "Loading configuration from #{config_file}" if verbose?
-      config = YAML.load(ERB.new(IO.read(config_file)).result(binding))
-      unless config
+      cfg = YAML.load(ERB.new(IO.read(config_file)).result(binding))
+      unless cfg
         $stderr.puts "Unable to parse config #{config_file}" if verbose?
         return {}
       end
-      convert_keys(config)
+      convert_keys(cfg)
     end
 
     def convert_keys(hash)
@@ -95,4 +95,4 @@ def default_configuration_paths
       ['~/.engine_cart.yml', '.engine_cart.yml']
     end
   end
-end
\ No newline at end of file
+end
diff --git a/lib/engine_cart/fingerprint.rb b/lib/engine_cart/fingerprint.rb
new file mode 100644
index 0000000..d0d8a8a
--- /dev/null
+++ b/lib/engine_cart/fingerprint.rb
@@ -0,0 +1,96 @@
+require 'bundler'
+require 'digest/md5'
+
+module EngineCart
+
+  def self.fingerprint_current
+    (@fingerprint_proc || method(:default_fingerprint)).call
+  end
+
+  def self.fingerprint_saved
+    File.read(EngineCart.fingerprint_file)
+  rescue StandardError
+    nil
+  end
+
+  def self.fingerprint_save(fp)
+    fp ||= EngineCart.fingerprint_current
+    File.open(EngineCart.fingerprint_file, 'w') { |f| f.write(fp) }
+  end
+
+  def self.fingerprint_update
+    EngineCart.fingerprint = EngineCart.fingerprint_current
+    EngineCart.fingerprint_save EngineCart.fingerprint
+  end
+
+  def self.fingerprint_file
+    File.expand_path('.generated_engine_cart', EngineCart.destination)
+  end
+
+  def self.fingerprint
+    @fingerprint || EngineCart.fingerprint_current
+  end
+
+  def self.fingerprint=(fp)
+    @fingerprint = fp
+  end
+
+  def self.fingerprint_proc=(fingerprint_proc)
+    @fingerprint_proc = fingerprint_proc
+  end
+
+  def self.rails_fingerprint_proc(extra_files = [])
+    lambda do
+      EngineCart.default_fingerprint + EngineCart.rails_fingerprint(extra_files)
+    end
+  end
+
+  def self.rails_fingerprint(extra_files = [])
+    EngineCart.files_digest(EngineCart.rails_files + extra_files)
+  end
+
+  def self.rails_files
+    Dir.glob([
+      './db/migrate/*',
+      './lib/generators/**/**',
+      './spec/test_app_templates/**/**'
+    ])
+  end
+
+  def self.default_fingerprint
+    EngineCart.env_fingerprint + EngineCart.gem_fingerprint
+  end
+
+  def self.gem_fingerprint
+    EngineCart.files_digest EngineCart.gem_files
+  end
+
+  def self.gem_files
+    Dir.glob('./*.gemspec') + [EngineCart.bundle_gemfile, EngineCart.bundle_lockfile]
+  end
+
+  def self.env_fingerprint
+    {
+      'RUBY_DESCRIPTION' => RUBY_DESCRIPTION,
+      'BUNDLE_GEMFILE' => EngineCart.bundle_gemfile
+    }.reject { |k, v| v.nil? || v.empty? }.to_s
+  end
+
+  def self.bundle_gemfile
+   @bundle_gemfile ||= Bundler.default_gemfile.to_s
+  end
+
+  def self.bundle_lockfile
+    @bundle_gemfile ||= Bundler.default_lockfile.to_s
+  end
+
+  def self.files_digest(files)
+    files_digests = files.collect {|f| file_digest(f) }.join
+    Digest::MD5.hexdigest(files_digests)
+  end
+
+  def self.file_digest(f)
+    File.file?(f) ? Digest::MD5.hexdigest(File.read(f)) : ''
+  end
+
+end
diff --git a/lib/engine_cart/tasks/engine_cart.rake b/lib/engine_cart/tasks/engine_cart.rake
index e5a5061..65f6fdb 100644
--- a/lib/engine_cart/tasks/engine_cart.rake
+++ b/lib/engine_cart/tasks/engine_cart.rake
@@ -78,11 +78,7 @@ namespace :engine_cart do
 
   desc "Create the test rails app"
   task :generate, [:fingerprint] => [:setup] do |t, args|
-    original_fingerprint = args[:fingerprint]
-    args.with_defaults(:fingerprint => EngineCart.fingerprint) unless original_fingerprint
-
-    f = File.expand_path('.generated_engine_cart', EngineCart.destination)
-    unless File.exist?(f) && File.read(f) == args[:fingerprint]
+    if EngineCart.update? args[:fingerprint]
 
       # Create a new test rails app
       Rake::Task['engine_cart:create_test_rails_app'].invoke
@@ -106,7 +102,7 @@ namespace :engine_cart do
 
       Bundler.clean_system "bundle install --quiet"
 
-      File.open(File.expand_path('.generated_engine_cart', EngineCart.destination), 'w') { |f| f.write(original_fingerprint || EngineCart.fingerprint) }
+      EngineCart.fingerprint_update
 
       puts "Done generating test app"
     end
diff --git a/lib/generators/engine_cart/install_generator.rb b/lib/generators/engine_cart/install_generator.rb
index 4d857d7..8e1b753 100644
--- a/lib/generators/engine_cart/install_generator.rb
+++ b/lib/generators/engine_cart/install_generator.rb
@@ -32,7 +32,9 @@ def install_engine
       end
     end
 
-    def ignore_test_app
+    def ignore_test_app(app_path = nil)
+      app_path ||= EngineCart.destination
+
       # Ignore the generated test app in the gem's .gitignore file
       git_root = (`git rev-parse --show-toplevel` rescue '.').strip
 
@@ -40,10 +42,10 @@ def ignore_test_app
       return unless File.exist? File.expand_path('.gitignore', git_root)
 
       # If the directory is already ignored (somehow) don't worry about it
-      return if (system('git', 'check-ignore', TEST_APP, '-q') rescue false)
+      return if (system('git', 'check-ignore', app_path, '-q') rescue false)
 
-      append_file  File.expand_path('.gitignore', git_root) do
-        "\n#{EngineCart.destination}\n"
+      append_file File.expand_path('.gitignore', git_root) do
+        "\n#{app_path}\n"
       end
     end
 
diff --git a/spec/integration/engine_cart_spec.rb b/spec/integration/engine_cart_spec.rb
index 14e0acb..99473bf 100644
--- a/spec/integration/engine_cart_spec.rb
+++ b/spec/integration/engine_cart_spec.rb
@@ -1,13 +1,21 @@
 require 'spec_helper'
 
 describe "EngineCart powered application" do
+
+  # EngineCart.destination can be defined by various config files and options.
+  # The engine_cart gem contains a .engine_cart.yml file, with destination: '.internal_test_gem'
+  let(:destination) { EngineCart.destination }
+
+  # The test app does not use a .engine_cart.yml file, so it uses a default destination: '.internal_test_app'
+  let(:default_destination) { EngineCart.configuration.default_destination }
+
   it "should have the test_app_templates pre-generated" do
     expect(File).to exist File.expand_path("spec/test_app_templates", EngineCart.destination)
   end
 
   it "should ignore the test app" do
     git_ignore = File.expand_path(".gitignore", EngineCart.destination)
-    expect(File.read(git_ignore)).to match /.internal_test_app/
+    expect(File.read(git_ignore)).to match /#{default_destination}/
   end
 
   it "should have a engine_cart:generate rake task available" do
@@ -27,27 +35,28 @@
   it "should create a rails app when the engine_cart:generate task is invoked" do
     EngineCart.within_test_app do
       `rake engine_cart:generate`
-      expect(File).to exist(File.expand_path('.internal_test_app'))
+      expect(File).to exist(File.expand_path(default_destination))
     end
   end
 
   it "should not recreate an existing rails app when the engine_cart:generate task is reinvoked" do
     EngineCart.within_test_app do
       `rake engine_cart:generate`
-      expect(File).to exist(File.expand_path('.internal_test_app'))
-      FileUtils.touch File.join('.internal_test_app', ".this_should_still_exist")
+      expect(File).to exist(File.expand_path(default_destination))
+      tmp_file_path = File.expand_path(File.join(default_destination, ".this_should_still_exist"))
+      FileUtils.touch tmp_file_path
       `rake engine_cart:generate`
-      expect(File).to exist(File.expand_path(File.join('.internal_test_app', ".this_should_still_exist")))
+      expect(File).to exist(tmp_file_path)
     end
   end
   
   it "should create a rails app when the fingerprint argument is changed" do
     EngineCart.within_test_app do
       `rake engine_cart:generate[this-fingerprint]`
-      expect(File).to exist(File.expand_path('.internal_test_app'))
-      FileUtils.touch File.join('.internal_test_app', ".this_should_not_exist")
+      expect(File).to exist(File.expand_path(default_destination))
+      FileUtils.touch File.join(default_destination, ".this_should_not_exist")
       `rake engine_cart:generate[that-fingerprint]`
-      expect(File).to_not exist(File.expand_path(File.join('.internal_test_app', ".this_should_not_exist")))
+      expect(File).to_not exist(File.expand_path(File.join(default_destination, ".this_should_not_exist")))
     end
   end