Upgrade python instead of the base image for python 3

[jsoriano]

Travis builds are taking too long in the python 3 branch after upgrading the Ubuntu image.
Keeping the image in trusty seems to speed up the builds. Timeouts disappear.
So instead of upgrading the image, upgrade python to 3.6 using deadsnakes PPA,
this is the minimal version for venv to work properly.
Python 3.7 doesn't work well because the trusty image in Travis has an old openssl version,
and Python 3.8 is not available for trusty.

Failures in tests after this change are consistent with failures in linux build in Jenkins.

[mikemadden42]

If this works, I'm fine with moving forward. We can revisit upgrading the version of Ubuntu at a future time.

[andrewkroh]

If this makes it stable then let's move forward. We can investigate the slowness issue with bionic independent of the Python upgrade.