The ImageMagick mirrors like to drop old tarballs, which means that our
formula breaks whenever the version we're using disappears.
So I've switched to using their SVN repo (and added a --HEAD build.)
Caveat: Their SVN repo is only served over https, with a bad cert,
so this brew know has a custom SVN download strategy that auto-accepts
that cert.
If this bothers you, get ImageMagick to fix their cert upstream (or
let us know where a stable tarball mirror lives.)
b2cde10e7e