Test against specific common Python versions #4
Description
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 |