Travis: add more revdeps on opium and slacko
Travis: add more revdeps on opium and slacko

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