Replace yamllint with python package since the ruby gem is not maintained

.gitlab-ci.yml

@@ -37,10 +37,11 @@ lint_ruby:
     - rubocop -l -f s
 lint_yaml:
   stage: linting
-  image: ruby:2.6-alpine
+  image: python:3.6
   script:
-    - gem install --no-document yamllint
-    - yamllint content/*.yaml
+    - pip3 install yamllint
+    - yamllint -v
+    - yamllint -s content/*.yaml
 rack_test_2-6:
   stage: testing
   image: ruby:2.6-alpine