Add Python 3.8 env in Travis CI test matrix.
At present the pinning of flake8 disallows running of lint on
Python 3.8 systems.
Update flake8 ignore-list to ignore W504 instead of W503, the PEP
guidance is that either is ok, but there must be local consistency.
There are more occurences of binary operator before line-break
than after in this repository, and we have also chosen to ignore
W504 in most of our other repositories.
Update installation sequence to fit the Travis Xenial environment.
This should save us some time on binary deps as we no longer need
to install snapd and kernel packages mandated by spectre/meltdown
in the Travis ``trusty`` image.
Chris MacNaughton
Frode Nordahl
Liam Young
Ryan Beisner