backwards comptatible get_queryset, for Django < 1.6

On newer versions of Django the runserver reports the deprecation warning. This uses the new standard and adds support for the existing one which older versions of Django till expect:

```
python2.7/site-packages/django/forms/widgets.py:142: DeprecationWarning: `VersionMetaAdmin.queryset` method should be renamed `get_queryset`.
  .__new__(mcs, name, bases, attrs))
```