Switch to protocol relative URLs (for the font imports)

The compiled .css files contained absolute URLs for the font
imports (e.g. "@import url(http://fonts.googleapis.com…"), which is
causing problems if the stylesheet is used on https:// sites, because
some browsers (e.g. Chrome), won't load non-secure content (i.e. the
http:// font imports) and also show a warning message.

We avoid this by substituting the font imports to protocol relative
links. Google also serves the font data via https://

It's always a good idea to use protocol relative URLs in code like
stylesheets or scripts.