Deployment User Setup

The `continuous_integration.rb` now uses the Travis CLI tool to generate SSH keys for the deployment user.
I don't think it's necessary to log out of Travis and log back in as the deployment user. Even though there's a one-to-one relation between GitHub users and Travis users, they aren't the same thing. The Travis tool only needs the deployment user's GitHub credentials to upload the public key to the correct GitHub account -- it doesn't need to be signed into Travis as the deployment user to upload the private key. Travis doesn't know or care who the private SSH key belongs to -- it only cares that the Travis user who uploads it authorized to do so.

As a result, the Travis CLI tool [always has to ask for GitHub credentials](https://gist.github.com/vandahm/88e3a67199899c7479c5#file-output-log-L7) when running `travis sshkey -g` and entering those credentials doesn't affect (and isn't affected by) any existing state.