Remove jekyll from gem install

Motivation:

A problem suddenly appeared where CI/CD jobs started to fail.

This is (seemingly) from the 'gem install' command pulling in a newer
version of jekyll, which caused an incompatibility with one of its
dependencies (jekyll --> sass-embedded).  Although the exact nature of
the incompatibility is unclear, the install fails while building, with
the error:

    NameError: uninitialized constant JSON::Fragment

Modification:

Remove jekyll from "gem install".  Use bundler to fetch jekyll instead.
This should honour the version.

Results:

More reliable builds

.gitlab-ci.yml

@@ -8,7 +8,7 @@ variables:
   JEKYLL_ENV: production
 
 before_script:
-  - gem install bundler jekyll --no-document
+  - gem install bundler --no-document
   - bundle install
 
 test_oai_pmh: