Add source map on debug mode with Sprockets 4

[GCorbel]

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This fix https://github.com/sass/sassc-rails/issues/161. With Sprockets 4, when debug mode was enabled, the source map wasn't added correctly at the end of the file.

[jrochkind]

This branch does make SCSS sourcemaps work for me with sprockets 4 (in Rails).

Without the fix in this branch, dev tools debugging of SCSS was pretty impossible in sprockets 4; I had to disable source maps in my browser. (The source maps resulted in unusable mapping, where Chrome just though every line of CSS came from an @import statement, and I couldn't figure out how to disable source map generation altogether for sprockets).

Thanks @GCorbel, amazing work.

I would love to see this merged and in a sassc-rails release, so I can re-enable use of source maps in Chrome.

(Still very mystified why this hasn't been a higher-profile issue; is nobody else using SCSS and sprockets 4? That seems hard to believe, as they are both standard installs in a new rails app. Mystified!)

[GCorbel]

@jrochkind thanks for your review. I applied your comments and did some refactoring.

[GregLMcDonald]

Would love to see this fix deployed in a new release. Debugging sass in a complex app is pretty painful as things stand.

[panozzaj]

I ran into this issue today, and fixed by using the code in this PR:

1. Made my Gemfile reference these commits:

gem 'sassc-rails', github: 'GCorbel/sassc-rails', branch: 'add-source-maps-on-debug'

2. Changed ./config/environments/development.rb to have:

  config.sass.inline_source_maps = true
  config.assets.debug = true

3. rm -rf ./tmp/cache based on the gem's README
4. Restarted my development server
5. Now the Chrome Inspector directly links to the SCSS source.

I would +1 merging and cutting a new release, as a result.

(I am not a maintainer of this project, just a user.)