Centers text in "remove ad" popovers

In reference to issue #3414. This was the best way I could target that specific popup. If there is a better way, let me know and I'll fix it.

Result: 
![3414](https://cloud.githubusercontent.com/assets/3391909/3362475/34caae94-fb0a-11e3-8580-0f7adef1a74d.jpg)