Don't include cache in serialization of backend

Fixes #227

imagekit/cachefiles/backends.py

@@ -1,4 +1,5 @@
 from ..utils import get_singleton, sanitize_cache_key
+from copy import copy
 from django.core.cache import get_cache
 from django.core.exceptions import ImproperlyConfigured
 
@@ -76,6 +77,13 @@ class CachedFileBackend(object):
         else:
             self.cache.set(key, state)
 
+    def __getstate__(self):
+        state = copy(self.__dict__)
+        # Don't include the cache when pickling. It'll be reconstituted based
+        # on the settings.
+        state.pop('_cache', None)
+        return state
+
     def exists(self, file):
         return self.get_state(file) is CacheFileState.EXISTS