Update _clearfix.scss to three-line modern clearfix

Use a more modern *three line* clearfix based on Nicolas Gallagher's 2011 micro clearfix, as announced and explained by Thierry Koblentz here: http://www.css-101.org/articles/clearfix/latest-new-clearfix-so-far.php.

This is used by Inuit.css, so it's certified legit.