travis: compile with both the newest and the oldest supported ocaml
travis: compile with both the newest and the oldest supported ocaml

mattam82/coq-opam-archive,ybertot/opam-coq-archive,coq/opam-coq-archive,ybertot/opam-coq-archive,gmalecha/opam-coq-archive,mattam82/coq-opam-archive,mattam82/coq-opam-archive,gmalecha/opam-coq-archive,ybertot/opam-coq-archive,gmalecha/opam-coq-archive