Revert "Merge pull request #106 from alphagov/bump-ruby-to-2.0.0"

This change is breaking the build.

This reverts commit 2a2b831df98e67e4e0ee4c10fcec5c80d126ac53, reversing
changes made to 1fea7aaee7904a2b598cc8a0a164d3d405888ac7.