Fix non-passing test

(because the test itself was making an extraneous assertion)

coin/billing/tests.py

@@ -316,7 +316,7 @@ class InvoiceQuerySetTests(TestCase):
     @freeze_time('2016-01-01')
     def test_number_workflow(self):
         iv = Invoice.objects.create()
-        self.assertEqual(iv.number, 'DRAFT-1')
+        self.assertEqual(iv.number, 'DRAFT-{}'.format(iv.pk))
         iv.validate()
         self.assertRegexpMatches(iv.number, r'2016-01-000001$')