Fix Django.utils.importlib deprecation warnings.

This changes `from django.utils.importlib import import_module` to
`from importlib import import_module`, as `django.utils.importlib` will
be removed in django version 1.9.

zerver/lib/socket.py

@@ -4,7 +4,7 @@ from six import text_type
 from typing import Any, Union, Mapping, Optional
 
 from django.conf import settings
-from django.utils.importlib import import_module
+from importlib import import_module
 from django.utils import timezone
 from django.contrib.sessions.models import Session as djSession