Fix MAC address formatting

MAC addresses are formatted in hex with colons.

```
% arp 192.168.128.79   
Address                  HWtype  HWaddress           Flags Mask            Iface
192.168.128.79           ether   08:00:28:58:6a:93   C                     wlan0
% tessel wifi -m
TESSEL! Connected to TM-00-04-f0009a30-00699742-58ce9249.
INFO Requesting MAC Address...
INFO Mac Address: 08:00:28:58:6a:93