update for new sprocket-rails which uses a json manifest file instead of a yml one.
Make it work with Rails 3 and 4
Fix deprecation warnings during test runs
Run tests on Rails 3.1, 3.2 and 4.0
Sprockets::Manifest does not respond to #[] any more

Use Sprockets::Manifest#assets instead
Merge pull request #1 from sauliusg/feature/rails4

Sprockets::Manifest does not respond to #[] any more
Reload the manifest to get newly compiled assets