Update import of importlib

Work around `RemovedInDjango19Warning`. Fixes #93

redis_cache/utils.py

 import warnings
 
 from django.core.exceptions import ImproperlyConfigured
-from django.utils.importlib import import_module
 from redis.connection import SSLConnection
 
 from redis_cache.compat import (
     smart_text, python_2_unicode_compatible, parse_qs, urlparse
 )
 
+try:
+    import importlib
+except ImportError:
+    from django.utils import importlib
+
 try:
     basestring
 except NameError: