Add a PEP8 check to travis.yml

[mbernier]

Issue Summary

According to codeClimate, this repo is not conforming with PEP8 standards. Please add a PEP8 check to Travis that will cause the build to fail if the proper rules are not being met.

Steps to Reproduce

1. Install the check
2. Make obvious PEP8 standards violation in one of the files
3. Make a PR
4. Let the build run
5. See a failure in the build
6. Fix the issue and make a comment (so we know)
7. Get a free Hacktoberfest shirt (so long as your first PR is in Oct)

[NdagiStanley]

• Install the check

• Make obvious PEP8 standards violation in one of the files

• Make a PR

• Let the build run

• See a failure in the build
  Here's the link to the build: https://travis-ci.org/sendgrid/python-http-client/builds/294729153

• Fix the issue and make a comment (so we know)

@mbernier The last step is being fixed by #64 , right?
Regardless, I have fixed the issue and we have successful builds now.

[NdagiStanley]

Secondly, from the travis build we see:

I have updated it from pep8 to pycodestyle.