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
