v3.21.0

This release has a fix that makes it possible to run new happo (this repo/library) with legacy happo (https://github.com/Galooshi/happo). Since they both use the same name for the main binary (happo) it's impossible to run both side-by-side. This release contains a fix that makes the happo-ci script use the build/cli.js script directly, instead of invoking it via $(npm bin)/happo.