Install jekyll in gitlab-ci.yml

.gitlab-ci.yml

 pages:
   stage: deploy
-  image: jekyll/jekyll
+  image: ruby:2.1
   script:
+  - gem install jekyll
   - jekyll build -d public
   artifacts:
     paths:

_config.yml

 name: Your New Jekyll Site
 markdown: redcarpet
-pygments: true
+highlighter: true