Travis: Since the Forge is down, switch to the OPAM mirror in test
Travis: Since the Forge is down, switch to the OPAM mirror in test

mor1/ocaml-cohttp,marklrh/ocaml-cohttp,dsheets/ocaml-cohttp,marklrh/ocaml-cohttp,fdopen/ocaml-cohttp,Drup/ocaml-cohttp,inhabitedtype/ocaml-cohttp,mor1/ocaml-cohttp,edwintorok/ocaml-cohttp,inhabitedtype/ocaml-cohttp,fdopen/ocaml-cohttp,edwintorok/ocaml-cohttp,marklrh/ocaml-cohttp,Drup/ocaml-cohttp,dsheets/ocaml-cohttp,edwintorok/ocaml-cohttp,inhabitedtype/ocaml-cohttp,mor1/ocaml-cohttp,fdopen/ocaml-cohttp,dsheets/ocaml-cohttp,Drup/ocaml-cohttp