Use atom/language-sass's Sass grammar

This works better than the old Ruby Sass.tmbundle we were pulling from
svn.textmate.org.

atom/language-sass also contains an SCSS grammar, but I didn't switch to
using that grammar because it isn't obviously better than our current
one and I'm not an SCSS expert.
Merge pull request #1860 from github/atom-sass-grammar

Use atom/language-sass's Sass grammar