Bump ruby verison to 2.3.1 in .travis.yml

Author: Yarden Bar

.travis.yml

@@ -1,6 +1,6 @@
 script: "bundle exec rake validate && bundle exec rake lint && bundle exec rake spec SPEC_OPTS='--format documentation'"
 language: ruby
 rvm:
-  - "2.1.6"
+  - "2.3.1"
 
 sudo: false