remove extra top margin on feed app preview tiles (bug 1109232)

## Before

![screen shot 2014-12-09 at 11 29 05 am](https://cloud.githubusercontent.com/assets/674727/5364184/991557f8-7f96-11e4-8032-38c2ac1cd6a7.png)

## After

![screen shot 2014-12-09 at 11 28 58 am](https://cloud.githubusercontent.com/assets/674727/5364183/9893924a-7f96-11e4-98c8-f6f8e21ab805.png)