Add Ruby 2.3.0 to .travis.yml

I wanted to consider removing 1.9.3 support, but according to
http://guides.rubyonrails.org/getting_started.html Rails itself still
supports 1.9.3 so we should probably continue to do so in Spring until
Rails drops it (i.e. in Rails 5).

Author: jonleighton

.travis.yml

@@ -6,6 +6,7 @@ rvm:
   - 2.0.0
   - 2.1.5
   - 2.2.0
+  - 2.3.0
 env:
   - RAILS_VERSION="~> 4.0.0"
   - RAILS_VERSION="~> 4.1.0"