Specify ruby version to fix build
Specify ruby version to fix build

This currently tries to build on CI against Ruby 1.9.1 which doesn't work as
it doesn't have Bundler (and is also really old).
We need to set it to something more modern. Bouncer and Transition are now at
2.3.0 so it make sense to keep this in sync.

alphagov/optic14n,alphagov/optic14n