Cohttp 0.12.0+ support

There's a new version of Cohttp coming in shortly (v0.12.0) that has a slightly different request API.  I've updated the constraints on the existing Facebook SDK packages in OPAM to use the older Cohttp, but a release with this change would be most useful.

you can get the latest cohttp via

opam remote add mirage git://github.com/mirage/mirage-dev