Fix width of mobile-sized ad

## Before

![screen shot 2014-09-08 at 10 48 43](https://cloud.githubusercontent.com/assets/74132/4183502/73afdafc-373d-11e4-9474-f95353fb47e9.png)

## After

![screen shot 2014-09-08 at 10 48 49](https://cloud.githubusercontent.com/assets/74132/4183504/76edb4a0-373d-11e4-8967-c1f819b4e7df.png)