Fix for Issue 5157

See https://github.com/spree/spree/issues/5157

This PR adds assets reference for spree/backend and spree/frontend to their own assets files so that it includes the default stylesheets and js when `bundle exec rake test_app` is run.

![image](https://cloud.githubusercontent.com/assets/4770544/4174712/dc83ec18-35a2-11e4-8e99-256a85a6e17d.png)