Always build findlib when brewing opam

This patch changes ocamlbrew so that findlib is considered a pre-requisite for building opam.

The motivation for this patch is that `ocamlbrew -r` failed on one of my machines that had an existing ocaml install on it.  `ocamlbrew -r` does not brew findlib, and so opam used the existing findlib command that was for the old, non-brewed version of ocaml.  Unfortunately, this led to a compilation error.