Update pi-gpio.js

Hi there,
I'm using this setup on raspberry pi http://nwazet.com/nwazet-pecan-pi-relays

So relays pins are maped in the following manner:

Relay # 1: GPIO 25
Relay # 2: GPIO 17
Relay # 3: GPIO 27
Relay # 4: GPIO 22
Relay # 5: GPIO 23
Relay # 6: GPIO 24

I've added missing pin 27 from your list. If you accept this commit how long before this appears on node npm official repository?

Regards
Tim