Puma Rack handler is required by Capybara

See: https://github.com/teamcapybara/capybara/blob/7d693f068c44f6a460336da70fb6e9e5f94f3db9/lib/capybara.rb#L450

actionpack/lib/action_dispatch/system_testing/server.rb

-require "rack/handler/puma"
-
 module ActionDispatch
   module SystemTesting
     class Server # :nodoc: