Enable `ruby@2.3.x` on CI
Enable `ruby@2.3.x` on CI

[Ruby 2.3.0 has been released][release], along with [support on
Travis][support].

This commit adds the version to the CI test matrix.

[release]: https://www.ruby-lang.org/en/news/2015/12/25/ruby-2-3-0-released/
[support]: https://twitter.com/travisci/status/680327961652899840

koryteg/glimmer-rails,thoughtbot/ember-cli-rails,koryteg/glimmer-rails,thoughtbot/ember-cli-rails,rwz/ember-cli-rails,koryteg/glimmer-rails,rwz/ember-cli-rails,thoughtbot/ember-cli-rails,rwz/ember-cli-rails,weggesser/ember-cli-rails,seanpdoyle/ember-cli-rails,weggesser/ember-cli-rails,seanpdoyle/ember-cli-rails,weggesser/ember-cli-rails,seanpdoyle/ember-cli-rails