update .travis.yml for master branch

Author: gfngfn

.travis.yml

@@ -8,5 +8,7 @@ sudo: required
 install:
   - wget https://raw.githubusercontent.com/ocaml/ocaml-travisci-skeleton/master/.travis-ocaml.sh
   - bash -ex .travis-ocaml.sh
+  - opam install -y menhir
+  - eval `opam config env`
 script:
   - make all