Make it possible to disable asset compilation

by setting `RAILS_NO_ASSETS` environment variable.

Currently it's only possible to disable asset precompilation by altering the repository. We have a project that is deployed to multiple heroku instances (in our case so that we can have separate NewRelic reporting) and we don't want to wait for assets to compile on the instances that never run web dynos. Since we want to run the same branch on all of our instances, we don't want to have to maintain a separate branch that has `manifest.yml` in it.