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! 🤖

[codecov]

Codecov Report

Merging #67 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #67   +/-   ##
========================================
  Coverage    59.44%   59.44%           
========================================
  Files           16       16           
  Lines          429      429           
  Branches        26       26           
========================================
  Hits           255      255           
  Misses         168      168           
  Partials         6        6

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[pyup-bot]

Closing this in favor of #70