No compass

This PR removes `Compass` as a dependency from `medium-editor` and uses `libsass` through `grunt-sass` instead. This enables us to keep 100% within the JS realm. Combined with `autoprefixer` Compass becomes redundant for this project and by ignoring old vendor-prefixes we can also enjoy a 54% total size reduction for the CSS files.

My original incentive for creating this PR was for me to get rid of the errors that `compass/css3` was missing when I built my `medium-editor` enabled project using `gulp-sass`.