Build: use ocaml 4.08 for the CI

.gitlab-ci.yml

-image: ocaml/opam2:ubuntu-18.04-ocaml-4.07
+image: ocaml/opam2:ubuntu-18.04-ocaml-4.08
 
 variables:
   GIT_STRATEGY: clone
@@ -13,6 +13,9 @@ stages:
 build_and_test:
   stage: build
   script:
+    - opam repo add official-repo "https://opam.ocaml.org"
+    - opam repository add official-repo --set-default
+    - opam update
     - opam depext dune ocamlfind
     - opam install dune ocamlfind num
     - make