Better spacing around ads

See: https://github.com/GigaOM/gigaom/issues/6414

### Here's an example before (production):

![before](https://cloud.githubusercontent.com/assets/430385/5461226/5efddc88-8537-11e4-931b-48472ce147b8.png)

### Here's the after:

![after](https://cloud.githubusercontent.com/assets/430385/5461228/6330b3d4-8537-11e4-8aaa-1d302041e8c7.png)