Switch to the python:3.6 image for CI, to make sure we get Python 3.6

.gitlab-ci.yml

@@ -1,7 +1,7 @@
+image: python:3.6
+before_script:
+  - pip install pipenv
+  - pipenv install
 test:
   script:
-    - apt-get update -qy
-    - apt-get install -y python-dev python-pip
-    - pip install pipenv
-    - pipenv install
     - pipenv run pytest