Edit instructions for Jekyll installation to avoid dependency error

[ArnNag]

gem install --user jekyll results in a depndency error as the dependency jekyll-sass-converter requires Ruby version 2.4.0 or above. The latest version of Jekyll that works is 3.8.7.

[cg505]

Thanks for reporting! Does gem install --user jekyll -v 3.8.7 work? We can update the docs to say that until we're able to update Ruby.

[ArnNag]

Yup, that's what worked for me.