Fix dropped import dependencies

Not sure if the `Sprockets.register_preprocessor` line still needs to be there, but apparently while Sprockets does respond to register_preprocessor, Rails 4.1 at least requires you register the processor on `app.assets`, not on `Sprockets`, else the ImportProcessor just isn't called and any `@import` dependencies are never added.

Fixes my bug #87 and probably #80 too.