Upgrade to Bootstrap 3.1

* Our SASS gem is now official
* Updates to bootstrap can be found at
https://github.com/twbs/bootstrap/releases/tag/v3.1.0
Upgrade SASS to 3.2.12

* http://sass-lang.com/documentation/file.SASS_CHANGELOG.html
Merge pull request #171 from alphagov/bootstrap-3-1

Upgrade to Bootstrap 3.1 and SASS 3.2.12