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.)
This was a bit of a throwaway comment to help us understand what this did - we can throw it out or make it a bit more descriptive. I only said "thing" a lot because I have no imagination whatsoever. :battery: :statue_of_liberty: :8ball: