This ensures that all our automated tests are using the same version.
I'm not sure it matters what version Prettier runs on, but thought it should be the same.
This is a down-grade, but it's better to match the version in production. The next step will be to upgrade again for production.
The default is to use the latest version. That's often fine because we
keep our version up-to-date but it may not always be. It's also great if
builds are re-producible.
