Fix Django19Warning : The django.contrib.admin.util module has been renamed

RemovedInDjango19Warning: The django.contrib.admin.util module has been renamed. Use django.contrib.admin.utils instead.

coin/billing/admin.py

@@ -5,7 +5,7 @@ from django.contrib import admin
 from django.contrib import messages
 from django.http import HttpResponseRedirect
 from django.conf.urls import url
-from django.contrib.admin.util import flatten_fieldsets
+from django.contrib.admin.utils import flatten_fieldsets
 
 from coin.filtering_queryset import LimitedAdminInlineMixin
 from coin.billing.models import Invoice, InvoiceDetail, Payment