This seems generally like a good idea given that we're making syntax changes to
formulae & are going to keep doing so for a little while yet. Taps may have moved
over to that syntax, which then causes tap failures if brew isn't up-to-date.
Should fix situations like https://github.com/Homebrew/homebrew-php/issues/3545.
If you're using e.g. a `/usr/local/homebrew` prefix then don't require
the `/usr/local/Cellar` to be manually created to avoid e.g.
`/usr/local/homebrew/Cellar` being used. Let's do all we can to let
people use this `Cellar` location as it means they can put their
repository wherever they like and still use all our bottles.
