Fix translation flash_batch_empty

Hi,

I found a little bug when you doesn't select an element for a batch action in the list, it shows flash_batch_empty instead of the translated message.

Here you can see the message with the actual SonataAdminBundle:
![sonata translation error](https://cloud.githubusercontent.com/assets/520893/3906518/af1cb2bc-22f3-11e4-9220-75c9607f5f86.png)

And with my fix:
![sonata translation error fix](https://cloud.githubusercontent.com/assets/520893/3906517/ae04a5a6-22f3-11e4-8d00-dca956f224e3.png)

The test related to this might be broken (CRUDControllerTest line 2489), i tried to fix it but didn't find a way. The value returned by $this->session->getFlashBag()->get('sonata_flash_info') is NULL instead of the translated message.