Use Rails 4.2 series for tests

This has the benefit of not skipping ActiveJob tests by default (only
included from Rails 4.2).

Author: David Rodríguez

Gemfile-rails-dependencies

@@ -19,7 +19,7 @@ when nil, false, ""
     # Rails 4+ requires 1.9.3+, so on earlier versions default to the last 3.x release.
     gem "rails", "~> 3.2.17"
   else
-    gem "rails", "~> 4.0.4"
+    gem "rails", "~> 4.2.0"
   end
 else
   gem "rails", version