Constraints on packages before Mirage 2 imports

See https://github.com/mirage/mirage-dev for the new packages
you can never have too much cohttp! fixed :)

> On 4 Nov 2014, at 09:00, Thomas Gazagnaire <notifications@github.com> wrote:
> 
> In packages/irmin/irmin.0.8.1/opam:
> 
> > @@ -22,6 +22,7 @@ depends: [
> >    "core_kernel" {>= "109.55.02"}
> >    "uri" {>= "1.3.12"}
> >    "cohttp" {>="0.10.0"}
> too many cohttp in that file
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/ocaml/opam-repository/pull/3001/files#r19791023>.
>