Fixed loaddata raising IOError

I tried exporting some images (using `FILER_DUMP_PAYLOAD=True`) and it gets dumped correctly.
However, when trying to import them back in a new project I get an `IOError`.
The problem is that the directory where to put the file does not exist (`filer_public` does not exist`).

The doc says:

If the directory filer_public is missing, django-filer rebuilds the file tree from scratch. This can be used to get rid of zombie files, such as generated thumbnails which are not used any more.

But it doesn't seem to be true.