[buildout] Pin django-post-office to below 3.3

More recent versions require jsonfied greater or equal to 3
which itself requires a newer django version.

common.cfg

@@ -15,7 +15,8 @@ django-rest-swagger = >2.1
 django-guardian = >=1.4,<2.0
 djangorestframework = >3.7
 django-activity-stream = >= 0.6.5
-django-jsonfield = >= 1.0.1
+django-jsonfield = >=1.0.1
+django-post-office = <3.3
 jsonfield = <3.0
 
 [scripts]