Treat -2 as a negative approval for expirations.

* jeepyb/cmd/expire_old_reviews.py(main): Previously only reviews
with a -1 vote were expired, so those with a -2 and no -1 would
remain in review indefinitely. This corrects the script to also
treat -2 as a negative vote for purposes of auto-expiring a change.

Change-Id: Ia447d7b11abafd9b3bb7c7696a7f47ea51bacaa1
Merge "Treat -2 as a negative approval for expirations."