Revert "Use sass compressor as the default CSS compressor"

This reverts commit 3eef1c7c5d0c4538efb9e061b6fe68011f0352b1.
The css compressor should be set in sass-rails plugin. Check
https://github.com/rails/sass-rails/commit/339529f9f6433047df6358c0439e6641cb74f045
for the fix.