Test against specific common Python versions

Improve travis to specifically test against these common versions of python:

|os|python|version|
|-|-|-|
|Ubuntu 16.04| python2 | 2.7.12 |
|Ubuntu 16.04| python3 | 3.5.2|
|Ubuntu 18.04| python2 | 2.7.15-rc1 |
|Ubuntu 18.04| python3 | 3.6.5 |
|macOS 10.13.16| python | 3.7.0 |