heads up: mirage-platform moving to ocaml-4.0 snapshot

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

Just a heads up: I'm updating the new Mirage platform Xen runtime up from 
3.12.1 to a 4.00.1 snapshot.  Thomas and OCamlPro have some significant 
performance improvements that make this worthwhile, so it makes sense to move.

I'll send out a mail when it's done, but it should be mostly invisible beyond 
requiring an OPAM switch to a new compiler.

-anil