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

merged