add ruby 2.3.1 to travis.yml

Signed-off-by: Patrick Münch <patrick.muench1111@gmail.com>

Author: atomic111

.travis.yml

@@ -6,6 +6,7 @@ rvm:
   - 1.9.3
   - 2.0.0
   - 2.2.0
+  - 2.3.1
 
 bundler_args: --without integration
 script: bundle exec rake