ImageMagick - use svn repo instead of tarballs

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.)

Library/Homebrew/download_strategy.rb

@@ -198,12 +198,16 @@ class SubversionDownloadStrategy <AbstractDownloadStrategy
     end
   end
 
+  def _fetch_command svncommand, url, target
+    [svn, svncommand, '--force', url, target]
+  end
+
   def fetch_repo target, url, revision=nil, ignore_externals=false
     # Use "svn up" when the repository already exists locally.
     # This saves on bandwidth and will have a similar effect to verifying the
     # cache as it will make any changes to get the right revision.
     svncommand = target.exist? ? 'up' : 'checkout'
-    args = [svn, svncommand, '--force', url, target]
+    args = _fetch_command svncommand, url, target
     args << '-r' << revision if revision
     args << '--ignore-externals' if ignore_externals
     quiet_safe_system *args