Update Jekyll

.gitignore

 .bundle
 .sass-cache
+.jekyll-cache
 Gemfile.lock
 _site
 *.gem

Gemfile

@@ -2,7 +2,7 @@ source "https://rubygems.org"
 ruby RUBY_VERSION
 
 # This will help ensure the proper Jekyll version is running.
-gem "jekyll", "3.8.5"
+gem "jekyll", "~> 4.2"
 
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem
 gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]