fixes issue #51

When using the 'mail' transport, you get an exception:

Argument 2 passed to Swift_Transport_SendmailTransport::__construct() must implement interface Swift_Events_EventDispatcher

This happens because the argument to the Transport service is inserted twice. Once in the Resources/config/swiftmailer.xml, and once in the PHP code in DependencyInjection/SwiftmailerExtension.php