Use ImproperlyConfigured exception from Django

Users should handle django.core.exceptions.ImproperlyConfigured better
then a custom exception with the same name

redis_cache/cache.py

 from django.core.cache.backends.base import BaseCache, InvalidCacheBackendError
+from django.core.exceptions import ImproperlyConfigured
 from django.utils.encoding import smart_unicode, smart_str
 from django.utils.datastructures import SortedDict
 
@@ -14,10 +15,6 @@ except ImportError:
         "Redis cache backend requires the 'redis-py' library")
 
 
-class ImproperlyConfigured(Exception):
-    "Django Redis Cache is somehow improperly configured"
-    pass
-
 
 class CacheConnectionPool(object):
     _connection_pool = None