Use Private Sprockets Fork

The sprockets maintainer yanked the sprockets version we use from rubygems.

![selection_187](https://cloud.githubusercontent.com/assets/206108/4875392/5023ec1c-6295-11e4-8038-b3bc51f4dfac.png)

It happened that I had the specific sprockets version at my harddrive.

We really should update sprockets, but until this happens, the fork can
be used.

It seems other people have this problem too ->https://community.openproject.org/topics/1348?page=1&per_page=200