Bump mentions to v0.0.7

Mentions should only happen on pages whose output extension is HTML. It caused `@media` queries to be output with HTML:

![screen shot 2014-06-23 at 5 32 15 pm](https://cloud.githubusercontent.com/assets/237985/3364319/dff0e0aa-fb1d-11e3-92a7-d299e5f0d171.png)

https://github.com/jekyll/jekyll-mentions/releases/tag/v0.0.7