When Celery CachedFileBackend used with filesystem storage (django.core.files.storage.FileSystemStorage), everything works fine.
But there are issues with storages.backends.s3boto3.S3Boto3Storage (and it's fix from #391), as well as with django_s3_storage.storage.S3Storage.
Exception was:
```
Traceback (most recent call last):
...
File "/src/django-imagekit/imagekit/cachefiles/__init__.py", line 131, in __bool__
existence_required.send(sender=self, file=self)
...
File "/libs/utils.py", line 380, in on_existence_required
file.generate()
File "/src/django-imagekit/imagekit/cachefiles/__init__.py", line 94, in generate
self.cachefile_backend.generate(self, force)
File "/src/django-imagekit/imagekit/cachefiles/backends.py", line 136, in generate
self.schedule_generation(file, force=force)
File "/src/django-imagekit/imagekit/cachefiles/backends.py", line 165, in schedule_generation
_celery_task.delay(self, file.generator, force=force)
...
File "/lib/python3.6/site-packages/kombu/serialization.py", line 221, in dumps
payload = encoder(data)
File "/lib/python3.6/site-packages/kombu/serialization.py", line 350, in pickle_dumps
return dumper(obj, protocol=pickle_protocol)
kombu.exceptions.EncodeError: can't pickle _thread._local objects
```
