Enable flake8 check in tox/TravisCI

[leplatrem]

No description provided.

[leplatrem]

Although the flake8 check is ran locally, it does not appear to be ran on TravisCI

[codecov-io]

Codecov Report

Merging #77 into master will decrease coverage by 0.28%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
- Coverage   94.53%   94.24%   -0.29%     
==========================================
  Files          12       12              
  Lines         841      834       -7     
==========================================
- Hits          795      786       -9     
- Misses         46       48       +2

Impacted Files	Coverage Δ
jose/backends/pycrypto_backend.py	97.11% <0%> (ø)	⬆️
jose/jwt.py	94.15% <0%> (-0.04%)	⬇️
jose/jws.py	94.11% <0%> (ø)	⬆️
jose/__init__.py	100% <100%> (ø)	⬆️
jose/utils.py	63.15% <100%> (-10.53%)	⬇️
jose/jwk.py	100% <100%> (+4.87%)	⬆️
jose/backends/__init__.py	55.55% <100%> (+5.55%)	⬆️
jose/backends/cryptography_backend.py	100% <100%> (ø)	⬆️
jose/backends/ecdsa_backend.py	98.66% <50%> (ø)	⬆️

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[mpdavis]

lgtm