GridFSLoader Bug

Modified the way to access the actual image since an update to the latest vendors brought up the following issue:

    Fatal error: Call to a member function getCollection() on a non-object in /vendor/liip/imagine-bundle/Liip/ImagineBundle/Imagine/Data/Loader/GridFSLoader.php on line 48

Tickets:
- Fixes liip/LiipImagineBundle#305