Update TravisCI scripts to test mirage-conduit on 4.03 and 4.04
Update TravisCI scripts to test mirage-conduit on 4.03 and 4.04

avsm/ocaml-conduit,mirage/ocaml-conduit,samoht/ocaml-conduit,samoht/ocaml-conduit,djs55/ocaml-conduit,djs55/ocaml-conduit