Explictly pull in the ActionCable engine if Rails version is 6 or higher

Author: petergoldstein

example_app_generator/spec/support/default_preview_path

@@ -26,6 +26,7 @@ require_file_stub 'config/environment' do
     require "action_controller/railtie"
     require "action_mailer/railtie" unless ENV['NO_ACTION_MAILER']
     require "action_view/railtie"
+    require "action_cable/engine" if Rails::VERSION::STRING >= '6'
 
     # Require the gems listed in Gemfile, including any gems
     # you've limited to :test, :development, or :production.