Use `sudo -u <username>` to change users

The principle motivation for this commit is that the command maps provided by capistrano/rbenv now work within `as()` session (previously they were dying because `RBENV_ROOT` wasn't being passed into the `sudo`'d environment).

IMO, this also fixes the now-closed capistrano/sshkit#125 (I've outlined in a comment there why I believe this is so, even in the absence of sudo documentation)