update sass and compass

This updates sass and compass to the latest versions. There is no real reason to do that other than staying up-to-date. I checked the changelogs ([sass](http://sass-lang.com/documentation/file.SASS_CHANGELOG.html)/[compass](http://beta.compass-style.org/CHANGELOG/)) and there was nothing too important for us.

The second commit replaces our own flexbox mixins by the new ones from compass.