You don't need `:paths` in this line: `validate` only takes custom methods to call, and not the field as `validates` does.

I'm starting to think that we shouldn't use custom methods for validations at all because of `validates`/`validate`, and should make all of them class-based as we have for [some already](https://github.com/alphagov/transition/tree/master/app/validators), which means that we'd only be using `validates` and would at least be consistent... That's way outside the scope of this PR, though.