Bump govuk_frontend_toolkit to 1.4.0

This brings the govuk_frontend_toolkit up to the latest version to
pick up lots of fixes and improvements.
Merge pull request #124 from alphagov/bump-govuk_frontend_toolkit

Bump govuk_frontend_toolkit to 1.4.0
Check SASS compiles as part of Jenkins test runner

Running asset precompile will catch SASS syntactic/reference errors earlier.
Merge pull request #128 from alphagov/run-asset-precompile-as-part-of-jenkins

Check SASS compiles as part of Jenkins test runner
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.