Fixed admin invoice USERNAME_FIELD AttributeError

Fixes the following error:

AttributeError at /admin/djstripe/invoice/
'Invoice' object has no attribute 'USERNAME_FIELD'
Request Method:	GET
Request URL:	http://url.com/admin/djstripe/invoice/
Django Version:	1.6.2
Exception Type:	AttributeError
Exception Value:	
'Invoice' object has no attribute 'USERNAME_FIELD'
Exception Location:	/app/.heroku/python/lib/python2.7/site-packages/djstripe/admin.py in customer_user, line 226
Python Executable:	/app/.heroku/python/bin/python
Python Version:	2.7.4