Test Python 3.8 in the CI, switch from xenial to bionic

[vmarkovtsev]

Update all our .travis.yml-s.

[vmarkovtsev]

scipy does not have the wheels for 3.8 yet, so while it is possible to request 3.8-dev in Travis, waiting 20 minutes for the build from source is impractical. This is blocked.

[zurk]

related: src-d/ml-core#34