ci: use ocaml 4.05 on CircleCI

Signed-off-by: Thomas Gazagnaire <thomas@gazagnaire.org>

circle.yml

@@ -16,7 +16,7 @@ dependencies:
   - brew update && brew upgrade
   - brew install wget ocaml opam dylibbundler
   - opam init --comp system -n https://github.com/ocaml/opam-repository.git
-  - opam switch system
+  - opam switch 4.05.0
   - opam pin add datakit-server.dev . -n
   - opam pin add datakit-server-9p.dev . -n
   - opam pin add datakit.dev . -n