Updated getServerCredentials() to handle multiple connections

I spent countless hours trying to figure out why I couldn't get Rocketeer to deploy to multiple servers. `artisan deploy:setup` was working fine, but `artisan deploy` was only using the first connection.

I boiled the issue down to `getServerCredentials()` in `BaseDeployCommand`, where it seemed that only one connection was supported (a loop was missing).

Deployment to multiple servers now work perfectly for me! - This should also resolve Issue #41

It doesn't seem like any tests needs to be updated in order to merge this PR.