Make GitHub ribbon link dynamically point to specific files


![pylons-rtd-pull-request](https://f.cloud.github.com/assets/1501525/791924/c0ff09f0-eb5c-11e2-8b55-1a372571c223.png)


### Related ###
- [Pyramid Issue #1042](https://github.com/Pylons/pyramid/issues/1042#issuecomment-20905058)
- [Read the Docs > GitHub Integration](https://docs.readthedocs.org/en/latest/features.html#github-integration)
- [GitHub Ribbons](https://github.com/blog/273-github-ribbons)
    - [PSD files](https://github.com/github/media/tree/master/ribbons)
    - [Collegiate Font](http://www.fontriver.com/font/collegiate/)