Change deprecated import of get_model.

mayan/apps/events/classes.py

@@ -1,6 +1,6 @@
 from __future__ import unicode_literals
 
-from django.db import models
+from django.apps import apps
 from django.utils.translation import ugettext_lazy as _
 
 from actstream import action
@@ -23,7 +23,7 @@ class Event(object):
         self.__class__._labels[name] = label
 
     def commit(self, actor=None, action_object=None, target=None):
-        model = models.get_model('events', 'EventType')
+        model = apps.get_model('events', 'EventType')
 
         if not self.event_type:
             self.event_type, created = model.objects.get_or_create(name=self.name)