OPAM and homebrew

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

If you're using MacOS X/Homebrew to install OPAM, you no longer need my 'tap' 
to install it, as it has been merged into mainline Homebrew now.

$ brew remove opam
$ brew untap mirage/ocaml
$ brew update
$ brew upgrade
$ brew install opam

...should do the trick and get you back on mainline.  I'm going to spend some 
time on the Mirage website today, so will do this update at the same time.

-anil