Update import from guardian

Get rid of deprecation warning triggered by the fact that assign has been renamed to assign_perm.

The actual warning is:
```
[...]/lib/python2.7/site-packages/guardian/shortcuts.py:97: DeprecationWarning: Shortcut function 'assign' is being renamed to 'assign_perm'. Update your code accordingly as old name will be depreciated in 1.0.5 version.
```

Since django-userena depends on v1.1 I think it's safe to update the import.