Fixed margins in nav bar on mobile

I fixed this issue in the [tip4commit repo](https://github.com/tip4commit/tip4commit/pull/26), but noticed it's the same over here.

Removed the margin-bottom rule from:

.nav-justified>li>a of the /assets/application-25b8d7709fc3c19033d1292decf2d537.css:3892 (line 3892)

to remove the empty space at mobile resolutions.

### Before
![Before](http://i.imgur.com/2egBnON.png)

### After
![After](http://i.imgur.com/YHL4TV8.png)

It would obviously be better to remove the original rule which sets the margin-bottom value to "5px" in the first place, depending on whether you prefer this style.