Update the task_check_interval_source reference

GitLab issue #617. Thanks to Lukas Gill (@lukkigi) for
the report and debug information.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/sources/models/base.py

@@ -234,7 +234,7 @@ class IntervalBaseModel(OutOfProcessSource):
         PeriodicTask.objects.create(
             name=self._get_periodic_task_name(),
             interval=interval_instance,
-            task='sources.tasks.task_check_interval_source',
+            task='mayan.apps.sources.tasks.task_check_interval_source',
             kwargs=json.dumps({'source_id': self.pk})
         )