Flexslider.css Firefox negative margin bug

In the latest Firefox if you're using negative margins on '.flexslider', plus a positive bottom margin it cause  flexslider to move down by whatever value is giving on the bottom margin. Changing the content value to a period fixes this and causes no harm to other browsers.
Merge pull request #998 from dulldave/patch-1

Flexslider.css Firefox negative margin bug - could probably be updated to micro clearfix.