- Ruby 2.4.9 is no longer supported, switch to latest
- Update the README to make the instructions copy/paste friendly
- Switch to latest Jekyll major/minor
- Update the `post_url` to not prepend `site.baseurl` as per
https://jekyllrb.com/docs/upgrading/3-to-4/. Note that since filters
were used and not path concatenation, this makes no difference for the
website but prevent further issues from being introduced.
- Update `.gitignore` to not track `.jekyll-cache`