+cohttp.0.9.15

Also fix constraints for previous versions
Merge pull request #1566 from mirage/master

+cohttp.0.9.15
Add some missing constraints typing core_extended.108.00.02 to the same version of Core
Merge pull request #1567 from mirage/master

Add some missing constraints typing core_extended.108.00.02 to the same ...
Merge branch 'master' of github.com:ocaml/opam-repository