Fix a typo in ruby test version

.travis.yml

@@ -12,7 +12,7 @@ matrix:
     script: bundle exec rake beaker
     services: docker
     sudo: required
-  - rvm: 2.2.1
+  - rvm: 2.3.1
     dist: trusty
     env: PUPPET_INSTALL_TYPE=agent BEAKER_debug=true BEAKER_set=ubuntu-16.04
     script: bundle exec rake beaker