This also reformats the YAML, for which I apologize, but it was hard to
work on it without doing that.

It drops dist: xenial which has become the default.

It renames matrix: to jobs: because that's the preferres spelling
according to Travis CI docs.

It adds names to all the special build jobs.

It moves regular Linux Python builds outside build matrix, for
conciseness (unfortunately this means 3.6 builds are now done twice:
once without -fwrapv and once with)

It moves docker pull and twine upload out of .manylinux.sh and into
.travis.yml.

It unifies twine upload calls by supplifying username/password via
environment variables.

The changes largely mirror those in
https://github.com/zopefoundation/BTrees/pull/115.