Update Readme w/ new Passenger Enterprise config sample.

The passenger enterprise gem server only allows authenticating with the
download token now. In addition to updating the Readme the ability to
use order reference/order password for the gem server has been disabled.
Merge pull request #208 from bryantraywick/203-update-readme-with-new-passenger-enterprise-config-sample

Update Readme w/ new Passenger Enterprise config sample.
Fixes #203