update the travis script for new opam release


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15023 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
better fix for the travis script


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15024 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02