Only support up to sass-rails 4.0.0

Compass Rails expects to monkey patch `sass-rails`'s `SassTemplate` in very specific ways that break on pretty much any patch release.

https://github.com/Compass/compass-rails/blob/master/lib/compass-rails/patches/sass_importer.rb

I would recommend completely locking down the supported patch version to ensure the patches are always applied correctly. Then advance the version only after testing.

I think this should help resolve upgrade pain when people try to upgrade either sprockets or sass-rails and everything breaks because of an out of date compass-rails patch.

Thanks!

/cc @rafaelfranca