Improve Gitlab CI

 + Add system compilers, much better for the ocaml_sys job, since opam does not
 need to spend long time installing a compiler for this job.

.gitlab-ci.yml

@@ -1,5 +1,5 @@
 before_script:
-  - apt-get update -qq && apt-get install -y -qq opam
+  - apt-get update -qq && apt-get install -y -qq opam ocaml ocaml-native-compilers
   - opam --version
 
 # OCaml version of the ci runner, i.e. from the system