Automatically trust Heroku's SSH key

This prevents the following scary "RSA key fingerprint" warning::

RailsBridge-VM:~/workspace/railsbridge/suggestotron$ git push heroku master
The authenticity of host 'heroku.com (50.19.85.156)' can't be established.
RSA key fingerprint is 8b:48:5e:67:0e:c9:16:47:32:f2:87:0c:1f:c8:60:ad.
Are you sure you want to continue connecting (yes/no)?
Set name and email in gitconfig

Without this, students cannot make any commits. They get this message:

$ git commit -m "Added a brand-new Rails app"

*** Please tell me who you are.

Run

  git config --global user.email "you@example.com"
  git config --global user.name "Your Name"

to set your account's default identity.
Omit --global to set the identity only in this repository.