Make the redirector build fail if any hosts contain uppercase characters

This is based on the comments on https://github.com/alphagov/transition/pull/445#discussion-diff-19723259.

(Paired with @rgarner on this.)
I'm also not sure that we ever use the right hand side of this `Hash`, which means we're using it as a `Set` (effectively, an `Array` with no duplicate values). We can say `uppercase = Set.new` instead to make that explicit.