README: Point at the OCaml PPA we actually use in the build

AVSM's PPA has moved on to provide a newer version of OCaml, which
then conflicts with the packages we build using an older OCaml in
the pbuilder chroot.

Works around issue #346

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #348 from euanh/master

README: Point at the OCaml PPA we actually use in the build