Update django-storages to 1.6.4

[pyup-bot]

There's a new version of django-storages available.
You are currently using 1.6.3. I have updated it to 1.6.4

These links might come in handy: PyPI | Changelog | Repo

Changelog

1.6.4

---

• Files uploaded with GoogleCloudStorage will now set their appropriate mimetype (320_)
• Fix DropBoxStorage.url to work. (357_)
• Fix S3Boto3Storage when AWS_PRELOAD_METADATA = True (366_)
• Fix S3Boto3Storage uploading file-like objects without names (195, 368)
• S3Boto3Storage is now threadsafe - a separate session is created on a
  per-thread basis (268, 358)

.. _320: jschneier/django-storages#320
.. _357: jschneier/django-storages#357
.. _366: jschneier/django-storages#366
.. _195: jschneier/django-storages#195
.. _368: jschneier/django-storages#368
.. _268: jschneier/django-storages#268
.. _358: jschneier/django-storages#358

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖