Tweak URL path prefix patch to address feedback from xaviershay

When reviewing my pull request, xaviershay requested these changes:

* Use ActionController::Base.config.relative_url_root instead of
  ENV['RAILS_RELATIVE_URL_ROOT'] in the UrlHelper and rackup.ru
* Mention the feature in the README, because it won't be found in
  config/application.rb

Implement these, but use Rails.configuration instead of
ActionController::Base.config, as it seems like the current Rails best
practice.