Bump to Jekyll 2.4.0

Release: https://github.com/jekyll/jekyll/releases/tag/v2.4.0
Diff: https://github.com/jekyll/jekyll/compare/v2.3.0...v2.4.0 (all the important stuff is in `lib/`, the rest is testing/documentation)
Co-sponsored by @gjtorikian and @kansaichris who put a LOT of work into this.

With love from SF.

/cc @benbalter @mastahyeti @ptoomey3 @gregose