Update gitlab ci configuration

Update Python version to 3.6 since most of our machine runs Python3.6 not 3.7 Update max line length to 79

.gitlab-ci.yml

@@ -13,3 +13,5 @@ linting:
     - lint
   script:
     - pylint --ignore=tests.py --max-line-length=79 *.py app
+  only:
+    - merge_requests