Cannot omit `model` field in ViewSet, when `serializer_class` has been defined. 

The `model` field should be allowed to omitted when `serializer_class` field exists. 

Currently, it will through an excpetion
```
'NoneType' object has no attribute '_meta'
Request Method:	GET
Request URL:	http://localhost:8000/api/parent/1/child/
Django Version:	1.6.2
Exception Type:	AttributeError
Exception Value:	
'NoneType' object has no attribute '_meta'
Exception Location:	/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/drf_toolbox/routers.py in get_lookup_regex, line 90
```