Add SSH key for heroku.com (per Gabe) to user's known_hosts

But without an IP address; whichever one is returned will be
automatically added by ssh when it connects.
Set user gitconfig push.default to simple
Give git default name and email (not a valid email address)

Students should still run the commands to configure git with their real
name before committing something they will push to Heroku, but this will
prevent an error message if they're experimenting before they do that.