updated omniauth-facebook, started using new FB API (no more usernames)

When I tried to authenticate against FB, we got errors due to the fact that FB introduced a versioned API, deprecated the old one, and in the new one stopped providing usernames in the oauth response.  

This PR:

1. updates the omniauth-facebook gem (handles some security issues, supports the versioned API, etc)
2. switches accounts to derive a username from a FB user's full name
3. configures omniauth-facebook to be tied to version 2.0 of the API (just to lock it down so things don't change out from under us again).