Adjust the bottom margin for tab dds.

:fork_and_knife: 

This fixes the style issue where a border-bottom was still displayed on active
tabs.

It was:

![screen shot 2014-06-03 at 11 51 06 am](https://cloud.githubusercontent.com/assets/928367/3162634/3cab44c6-eb37-11e3-9a82-04a2a775ff39.png)

It is now:

![screen shot 2014-06-03 at 11 52 27 am](https://cloud.githubusercontent.com/assets/928367/3162643/48a81664-eb37-11e3-80f0-69c40657562b.png)

Overriding an `!important` style with `!important` feels hacky but it gets the job done.