Corrected the hover style for buttons in the top-bar

I don't quite follow the JavaScript behaviour yet, but It seems that [this change](https://github.com/zurb/foundation/commit/68c529a24104ab6b1f13a301207039310d43b748) altered the way that the "hover" class is applied to the elements, and the SCSS rules seem to imply that the button should be given a **.hover** class.

This PR changes the SCSS to use the **:hover** rule and not require the JS at all. But I'm not sure if this is the intended behaviour.