Sassc/Scssc processors do not update imported files

[dagi3d]

I was testing the new libsass integration and found the following issue:

Given the following stylesheets
application.scss

@import '_base';

_base.scss

body {
  background: lime;
}

asset_pipeline:

Sprockets::Helpers.configure do |config|
  require 'sprockets/sassc_processor'
  assets.register_transformer 'text/scss', 'text/css', Sprockets::ScsscProcessor.new

If I make any change in _base.scss using the Scssc processor, the styles are not updated. To make the application.scss be updated, I need to make a change on it so it is precompiled again with _base.scss as well.
If I just the default scss processor it works without any problem.

[schneems]

I'm currently working on sassc support. There's some bugs. Right now i'm trying to get sassc-rails working with sprockets 4 sass/sassc-rails#65 after that i'm looking to get the SasscProcessor working and then hopefully get sass-rails to auto default to sassc when it's present and Sprockets 4 is being used. I don't have a timeline for those fixes but that's the gameplan.

[schneems]

Dupe of #329