Replace Atlas Cloud with Vagrant Cloud.
Created by: ladar
I updated the JSON and README file to say/use a Vagrant Cloud variable instead of Atlas, which was deprecated/decommissioned quite awhile ago.
Ideally the templates should also be able to use a VAGRANT_CLOUD_TOKEN environment variable, but I don't know off the top if it's possible to support both methods for setting an access token. With the environment variable, the access token parameter is unnecessary.