Add Ruby 2.3 in the test matrix

Author: rafaelfranca

.travis.yml

@@ -8,8 +8,9 @@ rvm:
   - 1.9.2
   - 1.9.3
   - 2.0.0
-  - 2.1
-  - 2.2.4
+  - 2.1.10
+  - 2.2.5
+  - 2.3.1
   - jruby-18mode
   - jruby-19mode
   - jruby-head
@@ -26,3 +27,4 @@ matrix:
   fast_finish: true
 script: bundle exec thor spec
 sudo: false
+cache: bundler