Amend Gitlab CI script

 + Forgotten "-y" option in  opam install

gitlab-ci.sh

@@ -12,7 +12,7 @@ opam --git-version
 ocaml -version
 
 # ocamlfind is mandatory to build
-opam install ocamlfind
+opam install -y ocamlfind
 
 # Building OcLaunch and running tests
 ./configure --enable-tests