Fix python test failure using virtualenv<20.0.0

[lucagiovagnoli]

Fixes #640 and #642 and improves tests running time from ~24min down to ~15min.

1. Fixes recent virtualenv==20.0.0 bug that broke Travis builds 3 days ago (ImportError: cannot import name 'ensure_text' pypa/virtualenv#1551).
2. Runs all python tests with all virtualenvs (for real! We were not running all of py27, py36, py37).
3. Python 2.7 is officially deprecated. Let me know if you'd rather remove this
4. Runs tests using Travis stages, so that we can run them in parallel and that we can actually use the correct system python version == tox python version. The problem seems to be that $TRAVIS_PYTHON_VERSION is unset (see Python tests are not run on all versions py27, py36, py37 #642). It looks like other projects on Github are using Travis stages to install the correct python version in each VM that is compatible to a specific tox environment.

One thing to note, unfortunately, due to some python tests relying on these jars: https://github.com/combust/mleap/blob/master/mleap-spark-extension/build.sbt#L9
I am forced to run sbt +compile before python tests in all stages (running in different VMs) and this makes them all slower because they are all compiling the entire project.

@talalryz @lydian @voganrc for review

[lucagiovagnoli]

scala tests and py36 passes.
sbt +compile fails in py37 tests. I wonder if that's because I changed the linux distro to xenial (as py37 is only available for xenial: https://travis-ci.community/t/unable-to-download-python-3-7-archive-on-travis-ci/639/2).

I could also try py35 and py38. We're not running on py37 right now anyways.

EDIT: this was because xenial is using java11 by default. Setting it to java8 fixes the problem

[talalryz]

Looks good to me!!

Thank you for fixing these issues!