Add missing Travis-CI python 3.6 support

[duboviy]

Build (all python interpreter versions) passed, following link provided below: https://travis-ci.org/duboviy/python-http-client

[Additional Note]: Python 3.7 can't be added right now due to Travis-CI issue: https://github.com/travis-ci/travis-ci/issues/9815 Python 3.7 could be added now only with dirty workaround: dist: xenial & sudo: true need to be specified (I'd suggest to wait https://github.com/travis-ci/travis-ci/issues/9815 fixed and add 3.7 normally in separate PR).

[SendGridDX]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #89 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #89   +/-   ##
=======================================
  Coverage   91.61%   91.61%           
=======================================
  Files           6        6           
  Lines         298      298           
=======================================
  Hits          273      273           
  Misses         25       25

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[thinkingserious]

Hello @duboviy,

Thanks again for the PR!

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Team SendGrid DX

[duboviy]

Hello,

Done. Thanks a lot!

Have a nice day, @thinkingserious!