Add a test job

.gitlab-ci.yml

 image: ruby:2.3
 
+test:
+  stage: test
+  script:
+  - gem install jekyll
+  - jekyll build -d test
+  artifacts:
+    paths:
+    - test
+  except:
+  - master
+
 pages:
+  stage: deploy
   script:
   - gem install jekyll
   - jekyll build -d public