Increase release message z-index to top-trump .ad-slot

Our [`.ad-slots`](https://github.com/guardian/frontend/blob/master/common/app/assets/stylesheets/module/_adslot.scss#L2) have a z-index of 1000, this ensures the release message sits on-top of them.

(Yes its already an extremely high number, but ad-slots need that for expandables to work correctly)

![google chrome](https://cloud.githubusercontent.com/assets/80089/3978536/02fe3874-2849-11e4-956c-6a37f41c8ea6.png)