require bundler groups to include rake-tasks in engines

If you generate a full engine, this will include rake tasks from
your gem under the `app` namespace. For example if you have a dependency
on `rspec-rails` in your engine's `gemspec`. You will get the task `app:spec`

Closes #8229

railties/CHANGELOG.md

@@ -1,5 +1,10 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Engines with a dummy app include the rake tasks of dependencies in the app namespace.
+    Fix #8229
+
+    *Yves Senn*
+
 *   Add sqlserver.yml template file to satisfy '-d sqlserver' being passed to 'rails new'.
     Fix #6882
 

railties/lib/rails/generators/rails/plugin_new/templates/rails/application.rb

@@ -11,7 +11,7 @@ require "action_mailer/railtie"
 <%= comment_if :skip_test_unit %>require "rails/test_unit/railtie"
 <% end  -%>
 
-Bundler.require
+Bundler.require(*Rails.groups)
 require "<%= name %>"
 
 <%= application_definition %>