should work with rails 4

- added shims for the asset helpers, but these will hopefully be addressed
- pull request on rails (https://github.com/rails/rails/pull/8681)
- pull request on sprockets-rails (https://github.com/rails/sprockets-rails/pull/33)
- issue opened on sprockets-rails (https://github.com/rails/sprockets-rails/issues/32) -- though I suspect it's an issue in rails.
- the shims that we've been adding can cause issues for people who prefix their assets differently.