Fix prior commit to fix widgy owner for python 2.6

So changing to `'admin:{}_{}_change'.format(self.object._meta.app_label, self.object._meta.module_name)`, messed up the tests for python 2.6. 

The join method is uglier than this alternative:

`'admin:%s_%s_change' % ( obj._meta.app_label, obj._meta.module_name)`

however, is this method an old style of python?

@gavinwahl, how would you like this to look?