Update .gitlab-ci.yml

.gitlab-ci.yml

 image: ruby:2.3
+# define the environment variables
+variables:
+  JEKYLL_ENV: production
+
+# 'bundle' all the required gems for the project
+before_script:
+  - bundle install
+
+# build and deploy the site
 pages:
-  stage: deploy
   script:
-  - bundle install
-  - JEKYLL_ENV=production bundle exec jekyll build -d public
+  - bundle exec jekyll build -d public
   artifacts:
     paths:
     - public