Fetch bootstrap-sass from rubygems instead of github

The bootstrap-sass gem now uses twbs/bootstrap version 3 by default and
no longer in release candidate status. Also, it was breaking when I tried to bundle install on my machine.
![opendevdata](https://f.cloud.github.com/assets/2944985/1773680/8fb4e1d6-67ed-11e3-8e54-7daa7dbff123.png)