Use tagged shell runner for GitLab CI

.gitlab-ci.yml

 stages:
   - lint-yml
 
-image: python:3.6
-
 variables:
   PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
 
@@ -13,6 +11,8 @@ cache:
     - apt-cache/
 
 lint-yml:
+  tags:
+    - shell
   stage: lint-yml
   before_script:
     - pip install -r test-requirements.txt