Set `push.default = current` in gitconfig

When students push to Heroku with `git push heroku` (vs `git push heroku
master`), there's a very scary error message about "push.default is unset".

Now when students forget to add "master" to the end of the command, it will Just
Work.
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.
Explain how to apply changes to local Vagrant box
Link to actual Installfest URL
Tell people not to worry about red text