Test against Python 3.6

[mewwts]

Description

Add Python 3.6 to build matrix in .travis.yml.

Motivation and Context

I want to run Luigi on Python 3.6. I have limited insight to tox, so I've made a basic attempt, but it might fail.

Have you tested this? If so, how?

Hoping travis will test this.

[mention-bot]

@mewwts, thanks for your PR! By analyzing the history of the files in this pull request, we identified @freyes, @Tarrasch and @gpoulin to be potential reviewers.

[mewwts]

The TOXENV=py33-nonhdfs is failing:

ERROR: InvocationError: '/home/travis/build/spotify/luigi/.tox/py33-nonhdfs/bin/coverage run test/runtests.py -v --ignore-files=(^\\.|^_|^setup\\.py$)'

But I think this is unrelated to py36?

[Tarrasch]

LGTM.

I feel a bit bad that we've not separated out the hdfs part and run all those tests on all changes. We're clearly wasting Travis's resources. But this PR is not the place to fix that. So LGTM.

[erikbern]

nice!