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/
