Update to use libsass 3.0

No issue

This doesn't give us any advantages at the moment, but is less buggy (in edge cases) and ;gets us be more awesome in the future.

Look at https://github.com/sass/libsass/releases/tag/3.0 to see what's in Libsass 3.0
Merge pull request #4296 from PaulAdamDavis/update-libsass

Update to use libsass 3.0