Make the config object lazy for old Djangos.

This should prevent import time side effects from instantiating the config object directly there.

constance/__init__.py

@@ -1,10 +1,12 @@
 from .base import Config
+from django.utils.functional import SimpleLazyObject
 
 __version__ = '1.0a1'
 
+
 try:
     from django.apps import AppConfig  # noqa
 except ImportError:
-    config = Config()
+    config = SimpleLazyObject(Config)
 else:
     default_app_config = 'constance.apps.ConstanceConfig'