Add six as a dependency.

[mblayman]

The six library is required, but it was not included in the packaging.

Fixes #389

[codecov-io]

Codecov Report

Merging #390 into master will decrease coverage by 0.55%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #390      +/-   ##
==========================================
- Coverage   93.13%   92.57%   -0.56%     
==========================================
  Files          52       56       +4     
  Lines        2693     2921     +228     
==========================================
+ Hits         2508     2704     +196     
- Misses        185      217      +32

Impacted Files	Coverage Δ
setup.py	80% <ø> (ø)	⬆️
rest_framework_json_api/__init__.py	100% <100%> (ø)	⬆️
example/api/serializers/__init__.py	78.18% <0%> (ø)
example/migrations/__init__.py	100% <0%> (ø)
example/models.py	95.6% <0%> (ø)
rest_framework_json_api/exceptions.py	84% <0%> (ø)

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