Feature/93 deploy to pvt GitHub repos

Allows deployManagedBeta to work with private GitHub repos when the following environment variables are present, otherwise it assumes that you are connecting to a public repo:

GITHUB_TOKEN  // Your GitHub username and password: e.g. <username>:<password>
GITHUB_OWNER // The name of the your account: e.g. SalesforceFoundation
GITHUB_REPO // Your repository name: e.g. CumulusCI