Upgrade sass to the lastest version

I have run the precompile with the old version and the new version. The
differences in our compressed css are as such:

- Spaces added between font family names
- Colour words converted to hex: 'white' -> '#fff' which is shorter
- Space added before '!important'
Merge pull request #486 from alphagov/upgrade-sass

Upgrade sass to the lastest version