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.
Fix typo and clarify
Set sequence.editor to nano just in case someone wants to rebase
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.
Merge pull request #16 from railsbridge-boston/git-user-friendlier

Git user friendlier
Explain how to apply changes to local Vagrant box
Link to actual Installfest URL
Tell people not to worry about red text
Remove hard linebreaks
Edit README, make rebuilding its own section, don't use rake for vagrant up