split out lint to a separate Travis job
    
    In order to speed our Travis CI gating even further, the lint step has
    been split to a separate job that can be run in parallel with the test
    runs. The test runs are in turn launched in developer mode to speed them
    up.
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>