No compass

This PR removes Compass as a dependency from medium-editor. 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 using autoprefixer 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`.