Added Travis job to run 'python3.6 setup.py test' instead of tox

[javabrett]

This adds a third build to the Travis matrix, to run python3.6 setup.py test instead of tox.
This will catch installation issues of the type seen in #403.

[sigmavirus24]

I'd rather not. setuptools commands are janky and often end up necessitating ridiculous amounts of debugging when something goes awry. No one presently involved in this project has the time or interest to do that. Also tox should be installing httpbin via the setup.py each time. That makes me skeptical that this would catch the issue.

[javabrett]

Also tox should be installing httpbin via the setup.py each time.

Is it currently only doing that for release builds?:

https://github.com/requests/httpbin/blob/master/tox.ini#L14
https://github.com/requests/httpbin/blob/master/tox.ini#L5

[testenv]
commands=python test_httpbin.py

[testenv:release]
...
commands =
    python setup.py sdist bdist_wheel
...

I'm not familiar with tox much, but maybe that is why it didn't catch this between releases ... was running python test_httpbin.py, should it be running python setup.py?

[sigmavirus24]

Tox by default runs python setup.py install