Apply 2.3.1 update

Updated to Bootstrap v2.3.1, trying to do it the "right" way:

  1. Modify `update-bootstrap.sh` to use a different git repo for `sass-twitter-bootstrap` (a fork that has been updated to 2.3.1), but make it pretty obvious and easy to change it back in future.
  1. Run the script
  1. Bump version in `bootstrap-sass.gemspec`