"arc upgrade", to automatically upgrade arc (client changes)

Summary:

• Try to limit the pain of future version bumps by making arc self-updating.
• When the server needs a newer version, prompt the user to update.
• (We need them to reissue their command because we may already have loaded classes which have changed in the update.)
• Make the message sound exciting!

Test Plan: Artifically bumped server forward, ran "arc list", got to upgrade!

Reviewers: Makinde, nh, jungejason, btrahan

Reviewed By: nh

CC: aran

Differential Revision: https://secure.phabricator.com/D2435