nope that was the old syntax, maybe this will work

.gitlab-ci.yml

@@ -7,8 +7,8 @@ build-frontend:
   image: node:latest
   tags:
     - docker
-  only:
-    - develop
+  rules:
+    - if: '$CI_COMMIT_BRANCH == "develop"'
   before_script:
     - npm i -g elm sass
   script:
@@ -27,8 +27,8 @@ build-binaries:
   image: ocamlpro/ocaml:4.12
   tags:
     - docker
-  only:
-    - develop
+  rules:
+    - if: '$CI_COMMIT_BRANCH == "develop"'
   before_script:
     - sudo apk add bash openssl-libs-static
   script: