Bump ruby to 2.1.2

This picks up latest optimisations.
This change also involves bumping the `sass` gem version to the latest (3.3.9)
because asset compilation fails under ruby 2.1.2 with the old version of the gem.
Merge pull request #129 from alphagov/bump-ruby-to-2.1.2

Bump ruby to 2.1.2