This change allows Jekyll to build presentation content outside the GitHub Pages environment, for example on Netlify.

Per https://www.netlify.com/blog/2017/05/11/migrating-your-jekyll-site-to-netlify

> If you have generated your Jekyll site using GitHub pages or forked a template there is a good chance you do not have a Gemfile. This is because GitHub pages will infer dependencies for you. If you would like to build your project outside of GitHub a Gemfile is needed and simple to create.

https://bundler.io/rationale.html

> Starting May 1st, 2016, GitHub Pages will only support kramdown, Jekyll’s default Markdown engine
– https://github.blog/2016-02-01-github-pages-now-faster-and-simpler-with-jekyll-3-0

GitHub Pages currently runs on Jekyll 3.7.4:
https://pages.github.com/versions