Fix Gitlab CI

 + Using yum with centos, since it works with old versions

.gitlab-ci.yml

@@ -69,7 +69,7 @@ debian:
 
 centos:
   before_script:
-    - sudo dnf install m4
+    - sudo yum install m4
   stage: real
   image: ocaml/opam:centos
   script: "export OC_NOTEST=true; ./gitlab-ci.sh system"