Install passenger enterprise from Phusion server.

Added option to install passenger enterprise from Phusion's gem server.
Set the `:order_reference:` and `:order_password:` settings in the
`:passenger:` section of `config/moonshine.yml` to install form the gem
server rather than a local gem file.
Updated Readme for passenger enterprise gem server
Merge pull request #162 from bryantraywick/install-passenger-enterprise-gem-from-phusion-passenger-gem-server

Install passenger enterprise from Phusion server.