TST: Run ASV on Travis?
Running ASV locally entrusts it to the pull requester, which means it gets run only in occasional circumstances (and is a bit of a burden).

Is there a way to run it on Travis, without significantly slowing down the builds? I know CircleCI has the ability to skip tests depending on the commit message - is there something similar we could do for Travis, so it only runs with an `#run_asv` string in the commit message (or similar)?