Update _clearfix.scss to use 'nano' 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](https://github.com/csswizardry/inuit.css/pull/133), which is a massively popular framework [used by huge projects such as BSkyB](https://github.com/csswizardry/inuit.css#as-used-by) - it's certified legit.

As well as being tiny, it's also less hacky, freeing up the `:before` element for styling.