Fix #311 ModelForm without either fields or exclude is deprectated

Author: davidfischer-ch

djcelery/admin.py

@@ -261,6 +261,7 @@ class PeriodicTaskForm(forms.ModelForm):
 
         class Meta:
             model = PeriodicTask
+            exclude = ()
 
         def clean(self):
             data = super(PeriodicTaskForm, self).clean()