Don't resolve AAAA records if there is no dnspython nor IPv6 support

If system doesn't has IPv6 support or dnspython package, socket.getaddrinfo 
with AF_INET6 flag return weird IP info for requested host, making SleekXMPP 
crush with more weird error.
Merge pull request #253 from kxepal/patch-1

Don't resolve AAAA records if there is no dnspython nor IPv6 support