travis: Switch to Docker

This runs parallel builds on docker, based on debian:testing
docker images, with ccache enabled and parallel running of the
test suite (ccache supports coverage builds these days, and
parallel testing just needs unbuffer to handle the stty stuff)

This is a huge step up from building on a mix of trusty, wily,
and xenial. Most importantly, coverage now correctly detects
partial covered lines instead of just reporting almost all of
them as fully covered.

We use ftp.de.debian.org as deb.debian.org backed by Amazon
503s too often.

Gbp-Dch: ignore

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