@font-face added to css minifiers

Recently [css-stringify added @font-face support](https://github.com/reworkcss/css-stringify/commit/a7fe6de82e055d41d1c5923ec2ccef06f2a45efa)

Meteor devel branch will fail to run/deploy if you have @font-face in your CSS without this bugfix.
It simply adds font-face support to the minification script.

It's also related to this [issue](https://github.com/meteor/meteor/issues/2028)