drop ipv6 on freebsd, to fix OSError 22

Traceback (most recent call last):
  File "eloipool.py", line 903, in <module>
    BitcoinLink(bcnode, dest=config.UpstreamBitcoindNode)
  File "/.storage/works/eloipool/bitcoin/node.py", line 50, in __init__
    sock.connect(dest)
OSError: [Errno 22] Invalid argument