Fix factory boy dependency

[santiavenda2]

Factoy Boy 2.9.+ versions are raising this problem when we run the test suite using py.test

ConftestImportFailure: (local('/home/santiago/develop/invgate-django-rest-framework-json-api/example/tests/conftest.py'), (<type 'exceptions.AttributeError'>, AttributeError("'DjangoOptions' object has no attribute 'postgen_declarations'",), <traceback object at 0x7fb1c6b123b0>))

Related issue pytest-dev/pytest-factoryboy#47

To fix this problem temporally we can switch to FactoryBoy 2.8.1.

[codecov-io]

Codecov Report

Merging #373 into develop will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #373      +/-   ##
===========================================
+ Coverage       93%   93.05%   +0.05%     
===========================================
  Files           51       51              
  Lines         2659     2665       +6     
===========================================
+ Hits          2473     2480       +7     
+ Misses         186      185       -1

Impacted Files	Coverage Δ
setup.py	80% <ø> (ø)	⬆️
example/tests/integration/test_polymorphism.py	100% <0%> (ø)	⬆️
rest_framework_json_api/serializers.py	84.78% <0%> (+0.72%)	⬆️

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[mblayman]

Thanks @santiavenda2. Would you mind adding a comment with a link to the factory boy issue to the code and/or requirements.txt file so that future developers will be able to know when that dependency limitation can be removed in the future?

[mblayman]

Nevermind. I took care of adding the comment. Thanks for the fix! 👍

[santiavenda2]

Thanks @mblayman 👍