emphasize that the private ssh key needs to be used in .travis.yml

Feedback from a customer:

> 
I'm so used to providing GitHub with public keys (and I'd just added a new one to our newly-created travis-only user account) that I'd set up my source_key with that user's public key. Turns out if you do that you get prompted for a passphrase. ... Maybe emphasizing "private key" a bit more in the setup document would have helped me avoid scanning over it the first time.

I have also renamed the key from `id_mustache_power` to `id_username_travisci`

Feel free to amend!