Use rsplit instead of split

[bakwc]

This is required for ipv6 hosts

[coveralls]

Coverage increased (+0.09%) to 88.525% when pulling 55999cb on rsplit into b3389a2 on master.

[CyberDem0n]

I guess this PR was made to support ipv6 addresses, but it's not enough, because in the dns resolver code you are using socket.gethostbyname_ex function, which supports only ipv4.
socket.getaddrinfo supports both

[bakwc]

Thanks, I'll fix that one too.