build: pin Python version in Travis

Travis will be switching the default version of Python from 2.7 to 3.6.
Our configuration and build scripts are not quite ready for Python 3
yet, so pin the version of Python to 2.7.

PR-URL: https://github.com/nodejs/node/pull/27166
Refs: https://changelog.travis-ci.com/upcoming-python-default-version-update-96873


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>