[travis] bin is not cached, reinstall opam each time
[travis] bin is not cached, reinstall opam each time

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