fix multi-app tiles errant height (bug 1058460)

The margin-bottom was being used as part of the parent `ul` height calculation and it serves no purpose as the `li` the `img` lives in has a fixed height.

![](http://f.cl.ly/items/2y3v1N1g320N3X2C2M06/Image%202014-08-28%20at%2011.09.18%20AM.png)