I added the gem, then at the behest of others because of whitehall's "gemits" :tm:  removed it and re implemented it here: https://github.com/alphagov/whitehall/commit/eac30f580c1f61065a40d8a13ab2d9cdab328ad4#L6R1

Rails does magic to pick up on the class name of a validator to allow you to use it as an arg to the validations.