[Certbot-dev] Faster Travis; merge master into your PRs

[Jacob Hoffman-Andrews]

In case you missed it, Brad and I sped up the Travis builds a lot by
removing the slowest tests: most of the Boulder integration ones, and
both of the macOS ones (https://github.com/certbot/certbot/pull/5270).
These tests now get run by Travis nightly on the "test-everything"
branch, so we will still get results from them.

If you have a large PR you're working on, I highly recommend you merge
master before your next push to get improved turnaround time on Travis.
It looks like Travis will usually complete in about 12 minutes now, down
from 2 hours 40 minutes last week.