default rails build to the latest version (#1770)

Author: JonRowe

Gemfile-rails-dependencies

@@ -25,7 +25,7 @@ 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", "~> 4.2.0"
+    gem "rails", "~> 5.0.0"
   end
 else
   gem "rails", version