Fix CLI team_id detection

Fixes the CLI ```--team``` issue @ksuther found in nomad/cupertino#83.  I didn't see any particular reason why we couldn't just use the ```team``` accessor instead of the instance variable, but my ruby-foo is weak.  Confirmed that this works for CLI and programmatic usage.

Alternatively, we could abandon the backwards compatibility and deprecate Agent::team_id, but that might require a major version bump.