Revert "Merge pull request #571 from alphagov/rails-upgrade"

Capybara-webkit 1.0 requires qt 4.8 which we don't have on the prod boxes.

This reverts commit eb1248fc5d4e2b9862c575477e72b977d1f13de4, reversing
changes made to efe50cd712056680bade47d28169ea5e9dffe8f2.