Update sass dependency

[MasterEnoc]

Sass gem is deprecated and should be updated to Sassc. To add the Sassc gem, sprockets needs to be updated to version 4.0.0 or greater and sprockets-sass is not needed since it does not support Sassc and has not been updated since 2016.

Sprockets 4.0.0 has changed the way to pre-compile files, looks like a 'manifest.js' file is needed to pre-compile files, at least with files that need two or more preprocessors, but I have not been able to make it work in this way.