Fix dependency to easy-extends-bundle

Fix issue #64 "Dependecy with easy-extends-bundle not documented".

I followed the installation documentation and got this error:
```
Generating autoload files
PHP Fatal error:  Class 'Sonata\EasyExtendsBundle\Mapper\DoctrineCollector' not found in 
.../vendor/sonata-project/notification-bundle/Sonata/NotificationBundle/DependencyInjection/SonataNotificationExtension.php on line 293
```