Use new factory service configuration

[pkruithof]

http://symfony.com/doc/current/components/dependency_injection/factories.html

Since 2.6, the factory-service and factory-method attributes have been replaced by a factory attribute. This raises deprecation warnings when, for example, clearing the cache:

[2015-07-25 12:58:27] php.DEBUG: The "factory_method" key of service "undeploy.taskrunner" in file "services.yml" is deprecated since version 2.6 and will be removed in 3.0. Use "factory" instead. {"type":16384,"file":"/home/vagrant/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php","line":210,"level":4352} 

Here's how Puli fixed it: puli/symfony-bundle@42b315c