Track chutney CI failures, and tweak the allow failures settings

CI failed one job on 0.2.9, but the debug run succeeded: https://travis-ci.org/torproject/chutney/jobs/505547963

We might need to increase the allowed failures for 0.2.9, or put a sleep in between the allow failures tests.

But we won't know which one to do until we see how CI works for a few days.