Updated Travis CI test code to additionally test against py3.6

[benosteen]

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Description of Change

Added an optional test stage to the Travis CI configuration that attempts to build and run the Arches test suite in a python 3.6 environment. Currently, as expected, it fails quickly but gives a test for developers a target to work towards passing.

Issues Solved

This goes someway to addressing part of #3165

Checklist

• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[coveralls]

Coverage remained the same at ?% when pulling 3e70e20 on 3165_python3_travis_ci into 51c7f78 on master.

[chiatt]

Thanks @benosteen - this will be very helpful