Fixes background icon alignment

Since some version of FontAwesome I don't recall, this center alignment is needed so the icons show centered on the container. I guess that when you forked the original dashing, you updated FontAwesome, but didn't apply this css fix.  The original Dashing repository applied this fix some time ago: https://github.com/Shopify/dashing/commit/3af326da84251069c4e6f7464d0ae8506b20e98b#diff-c05224104d6cdd7a84efb7ae4e8fd074L119