WIP: Update gitlab ci

• Update Python version to 3.6 since most of our machine runs Python3.6, not 3.7
• Update max line length to 79
• Set the stage to only run on merge requests

.gitlab-ci.yml

-image: "python:3.7"
+image: "python:3.6"
 
 before_script:
+  - env
   - python --version
   - pip install pylint
 
 stages:
-  - lint
+  - linting
 
 linting:
+  stage: linting
   tags:
     - lint
-  stage: lint
   script:
-    - pylint --ignore=tests.py --max-line-length=120 *.py app
+    - pylint --ignore=tests.py --max-line-length=79 *.py app
+  only:
+    - merge_requests