Removed isRequired() from Configuration (#300)

isRequired is forcing the user to specify the factory_class even when there is a default.

src/DependencyInjection/Configuration.php

@@ -116,7 +116,6 @@ public function getConfigTreeBuilder()
                                     ->end()
                                     ->scalarNode('factory_class')
                                         ->cannotBeEmpty()
-                                        ->isRequired()
                                         ->defaultValue(AmqpConnectionFactory::class)
                                         ->info('This option defines an AMQP connection factory to be used to establish a connection with RabbitMQ.')
                                     ->end()