travis: use tox, temporarily disable coverage
travis: use tox, temporarily disable coverage

I don't yet know how to get tox to run the equivalent of "coverage run
`which trial` wormhole.test", since tox doesn't use a shell. But I want
to switch travis to tox so we get the same dependencies (in particular
the pyflakes constraint) everywhere. I'll figure out how to re-enable
coverage data soon.

warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole