port should only be calculated once

Starting two process groups using the "PORT" env variable results in ports separated by 1000, although the forego log messages report the expect ports:

    forego start
    forego | starting web on port 5000
    forego | starting api on port 5100
    web    | 2013/10/23 09:30:57 listening localhost:5000
    api    | 2013/10/23 09:30:57 listening localhost:6000

Expected behavior is that the ports increment by 100.