Stop using netcat to forward SSH
Stop using netcat to forward SSH

New versions of SSH (5.4+) support the `-W` flag to forward
connections. Given Boxen requires Mountain Lion or above we can assume
this will work on our development Macs.

Thanks to @calpaterson for the original idea.

alphagov/gds-boxen,alphagov/gds-boxen,alphagov/gds-boxen