Publish a fedmsg message when a user's rank changes.

This requires fedora-infra/tahrir-api#19.

This is a fix for fedora-infra/fedbadges#19.
Oh, and tahrir-api will emit this message for us too using our notification_callback.
Remove explicit fedmsg initialization.  It is unnecessary.
Move notification_callback from a method to a function in fedbadges.utils.
Merge pull request #23 from fedora-infra/feature/publish-message-on-rank-change

Publish a fedmsg message when a user's rank changes.