Always set the new default when deploying.
Always set the new default when deploying.

Summary:
I think `--promote` is the default, so this probably works for most
people, but I had set a config to not promote by default so the newly
deployed version wasn't doing anything.  This fixes that.

Test Plan: Ran this command manually

Reviewers: benjaminpollack, benkraft

Reviewed By: benkraft

Differential Revision: https://phabricator.khanacademy.org/D33943

Khan/slack-deploy-hooks,Khan/slack-deploy-hooks