Fix bug for MongoDb Odb bundle 1.0.0-beta9

On Mongodb ODB bundle 1.0.0-BETA9, the lifecycle event does not have a
```getObject``` method (because it does not extends Doctrine\Common\Persistence\Event\LifecycleEventArgs)

See [the 1.0.0 file](https://github.com/doctrine/mongodb-odm/blob/1.0.0-BETA9/lib/Doctrine/ODM/MongoDB/Event/LifecycleEventArgs.php) and the [master version](https://github.com/doctrine/mongodb-odm/blob/master/lib/Doctrine/ODM/MongoDB/Event/LifecycleEventArgs.php)

When the next version is released you can revert this commit and use the
LifecycleEventArgs as an argument
Merge pull request #13 from jdeniau/master

Fix bug for MongoDb Odb bundle 1.0.0-beta9