Update .gitlab-ci.yml

Start to use cache (first try)

.gitlab-ci.yml

@@ -41,6 +41,9 @@ ocaml_403:
   image: ocaml/opam:alpine_ocaml-4.03.0
   script: "./gitlab-ci.sh"
   allow_failure: false
+  cache:
+    paths:
+      - /home/opam/.opam
 
 # OCaml version 4.03, with flambda optimisation
 ocaml_403_flambda: