Switch Travis builds to VM-based infrastructure
Switch Travis builds to VM-based infrastructure

See the following for more details:
- https://docs.travis-ci.com/user/reference/trusty#container-based-infrastructure
- https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures
jmluy/xpython,smalley/python,behrtam/xpython,exercism/xpython,behrtam/xpython,exercism/python,jmluy/xpython,exercism/python,exercism/xpython,smalley/python,N-Parsons/exercism-python,N-Parsons/exercism-python