Restrict ocamlnet 3.6.0, 3.6.3, and 3.6.5 to OCaml < 4.01.0

An error in https://godirepo.camlcity.org/wwwsvn/trunk/code/src/pop/netpop.ml?rev=1218&root=lib-ocamlnet2&view=markup (duplicate "method stat") triggers a new 4.01.0 error <http://caml.inria.fr/mantis/view.php?id=6035>. This causes many packages dependent on ocamlnet to fail to build under 4.01.0.

This pull request will be referenced from both the ocamlnet-devel list and the mantis issue's comments.