Use Travis's new container-based infrastructure
ActiveRecord 5 is Ruby 2.2 only, so don't test it w/ Ruby 2.1
Test on Rails 4.2 release, don't allow failure
Use Arel master when using ActiveRecord master

(Transplant from with_model/with_model@cab41ed225a5d1f1fb65e51edc5673f2583285bb)