Remove usage of tox

[kokosing]

tox requires that development workstation is equipped with two python installations (2.6 and 2.7). However it could be somehow difficult to achieve as some linux distribution no longer support usage of 2.6.

Notice that travis is able to test multiple version of python, see: https://docs.travis-ci.com/user/languages/python

[cawallin]

If Travis supports it, we can definitely use that; it's rare that there are issues with 2.6/2.7 support. But we can't stop testing Python 2.6 because that's the ancient version of Python that the Hadoop appliance has.

[ebd2]

I've been bit by 2.6/2.7 issues that tox has caught. I agree that we need to keep testing with 2.6, and tox is convenient for doing that locally.

I agree that if Travis has built-in tooling for testing with different versions, we should definitely use it.