Fix stylesheet asset expansion for Rails 4

- Rails 3 already worked incidentially (asset_for took an extension as a
  'hint', we've changed this to nil since it should only be necessary if
  you try to pass a file without an extension in jasmine.yml)
- Better usage of Sprockets methods to produce asset paths for Rails 4
- [fixes #54237304]

Conflicts:

	spec/jasmine_rails_spec.rb