[fix] ci: push-to-github only on master

.gitlab-ci.yml

@@ -19,7 +19,9 @@ push_to_github:
         - bash -c "cat packed-refs | grep -v 'refs/pull' > packed-refs-new; echo 'Removed pull refs.'"
         - mv packed-refs-new packed-refs
         - bash -c "git push --force --mirror github 2>&1 | grep -v duniter-gitlab; echo $?"
-      
+  only:
+    - master
+
 publish:  
   stage: deploy
   image: node:6.12-alpine