travis: run the test suite on various distributions

travis: run the test suite on various distributions

.travis.yml is obviously still Travis-specific, but tests/ci-* are
designed to be shareable with other CI environments if there is interest
in doing so.

At the moment I'm only testing on Debian and Ubuntu. In principle we
could try a non-Debian-derived Docker container such as Fedora or CentOS
inside travis-ci's Ubuntu environment, similar to what I'm doing
for Debian, but I don't know the correct setup commands to use there.

Closes: #438
Approved by: cgwalters