Give overlay higher z-index than advert

fixes https://github.com/guardian/frontend/issues/4472

### Before 

![screen shot 2014-05-29 at 14 17 31](https://cloud.githubusercontent.com/assets/74132/3117784/a1b2421e-e733-11e3-8e77-c9b414ff67af.png)

### After

![screen shot 2014-05-29 at 14 19 22](https://cloud.githubusercontent.com/assets/74132/3117805/de038ed0-e733-11e3-8dff-3eba9d6e3c88.png)