Downgrade Travis stem version to a commit where tests pass.
Due to https://github.com/torproject/stem/issues/63 our CI is failing. Let's downgrade to a working version of Stem unless it gets fixed right away.
I have a test PR at https://github.com/torproject/tor/pull/1889 ; if CI passes, I'll make PRs for the other branches and put this in needs_review.