`sudo: false` used to be the way to opt in to their container-based
infrastructure. That system was retired in 2018 and so the `sudo` option
no longer does anything. Remove it.
Travi CI supports multiple OSes now. Add `os: linux` to be explicit
about which OS we want to test on.
Markdown keeps breaking the 2.6 build. We could pin an earlier version,
or we can simply drop 2.6 support. The Python world is moving on; time
for us to do the same.
This will apparently allow us to build on Travis's new container-based build
system, which is supposed to lead to faster builds. The catch is that we
don't get root access, but we don't need it anyway.
http://docs.travis-ci.com/user/migrating-from-legacy/
