Minimize is incompatible with bundler versions after 1.12.

Results in an error like:
```
mimize.rb:31:in 'runtime_specs': undefined method 'dependencies_for' for #<Bundler::Runtime:0x005563ec212d98> (NoMethodError)
	from minimize.rb:38:in 'spec_git_files'
	from minimize.rb:54:in 'clean_git_files'
	from minimize.rb:66:in '<main>'
rake aborted!
```