Add python_requires to setup.py

[capsulecorplab]

Should only install for the uncommented versions in .travis.yml.
See https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

[Kilo59]

Should think about creating a travis-requirements.txt file, and maybe a Makefile with different logic if in Tavis CI environment.
Something like this...
https://github.com/CamDavidsonPilon/lifelines/tree/master/reqs
https://github.com/CamDavidsonPilon/lifelines/blob/master/Makefile