Drop support for python3.4, 3.5

[Kilo59]

To ease future development and especially to support making Polyglot Windows compatible in the future I would consider dropping support for Python3.4 and 3.5.

Specifically, a useful step would be to make failures in the Travis CI for Python3.4 and Python3.5 optional. See this example... https://github.com/CamDavidsonPilon/lifelines/blob/master/.travis.yml

Mostly I'm thinking about the improvements to the pathlib library in python 3.6 that should make compatibility on windows easier to implement if you don't have to worry about older versions of pathlib in 3.4/3.5.