Fix freezegun lifecycle

Should use decorator instead of manually start/stop to avoid failed setUp() leaving mock in place and breaking many
other tests.

tests/contributions/test_contributions_api.py

@@ -952,11 +952,10 @@ def test_rolefield_rate_interpretation(self):
         contribution.delete()
 
 
+@freeze_time('2015-09-25')
 class ReportingApiTestCase(BaseAPITestCase):
     def setUp(self):
         super(ReportingApiTestCase, self).setUp()
-        self.freezer = freeze_time("2015-09-25")
-        self.freezer.start()
         now = timezone.now()
         self.roles = {
             "FlatRate": ContributorRole.objects.create(name='Author', payment_type=0),
@@ -1144,7 +1143,6 @@ def setUp(self):
         Contribution.search_objects.refresh()
 
     def tearDown(self):
-        self.freezer.stop()
         super(ReportingApiTestCase, self).tearDown()
 
     def test_exclude_content_without_contributions(self):