Update Jekyll.gitlab-ci.yml to have parity with pages/jekyll

https://gitlab.com/pages/jekyll/blob/master/.gitlab-ci.yml

This also removes the comment about Jekyll version because this file does not specify a Jekyll version.  The version will be determined by the Gemfile[.lock] in the repository.

Pages/Jekyll.gitlab-ci.yml

 # Template project: https://gitlab.com/pages/jekyll
 # Docs: https://docs.gitlab.com/ce/pages/
-# Jekyll version: 3.4.0
 image: ruby:2.3
+variables:
+  JEKYLL_ENV: production
+
 before_script:
 - bundle install
@@ -25,4 +27,4 @@ pages:
     - public
   only:
   - master
-  
\ No newline at end of file
+