ping6 output on FreeBSD 12 is slightly different that base.py expects

[sobarber]

ping6 generates output that looks like this on FreeBSD 12:

PING6(56=40+8+8 bytes) 2607:fb90:9233:b34:2e0:67ff:fe2c:8518 --> 2607:f8b0:4023:1004::66
16 bytes from 2607:f8b0:4023:1004::66, icmp_seq=0 hlim=58 time=20.029 ms

--- google.com ping6 statistics ---
1 packets transmitted, 1 packets received, 0.0% packet loss
round-trip min/avg/max/std-dev = 20.029/20.029/20.029/0.000 ms

This means that line 52 in gateway/base.py needs to be altered to match 'std-dev' in this output.

pattern = b'(?:rtt|round-trip) min/avg/max(?:/(?:mdev|stddev))? = \d+.\d+/(\d+.\d+)/\d+.\d+(?:/\d+.\d+)? ms'

becomes

pattern = b'(?:rtt|round-trip) min/avg/max(?:/(?:mdev|stddev|std-dev))? = \d+.\d+/(\d+.\d+)/\d+.\d+(?:/\d+.\d+)? ms'