With this configuration, it always uses an updated version of ruby and bundler
is the one which manages version of the dependencies, hence working in all
environments. Before, dependencies could not be satisfied in updated
environments because of bundler being too new.
If a user is using GitLab Pages it is most likely the user is using GitLab pages to publish a website not just stage a website that is published elsewhere. If a user is publishing a website with Jekyll, they (and Jekyll themes/plugins) will expect that JEKYLL_ENV=production. This commit provides that specification in `.gitlab-ci.yml`.
An examination of the existing repository showed this should not result in any change to the current rendering of the website. However, the specification may be important for developers as explained in preceding commit.
