Jekyll on Heroku without Rack::Jekyll or custom buildpacks

config.ru

require 'rack/contrib/try_static'
require 'rack/contrib/not_found'

use Rack::TryStatic,
  :root => "_site",
  :urls => %w[/],
  :try  => ['index.html', '/index.html']

run Rack::NotFound.new('_site/404.html')

Procfile

web: bundle exec puma -p $PORT config.ru

Rakefile

namespace :assets do
  desc 'Precompile assets'
  task :precompile do
    sh "bundle exec jekyll build"
  end
end