Work out how to fail chutney travis jobs when the tor repository is out of date or missing

In #27912 (moved), we configure a bunch of deb lines that depend on deb.torproject.org. But what happens when the repository is updated after a release?

We can either:

• remember to update the travis config every release, or
• work out how to make the jobs fail when they fall back to 0.2.9.14 (or Ubuntu security updates' current version)

Maybe there's a travis option that we could use?

This task does not need to be solved straight away.