Change default IP address lookup URI

icanhazip.com has two other domains for fine-tuned IP lookup,
ipv4.icanhazip.com will always return the IPv4 address, and
ipv6.icanhazip.com will always return the IPv6 address (the latter
appears to only work if you actually have an IPv6 address).

Details at http://rackerhacker.com/icanhazip-com-faq/.

Closes #68.

powerline/ext/tmux/segments.py

@@ -24,7 +24,7 @@ def date(format='%Y-%m-%d'):
 
 
 @memoize(600, persistent=True)
-def external_ip(query_url='http://icanhazip.com/'):
+def external_ip(query_url='http://ipv4.icanhazip.com/'):
 	import urllib2
 	try:
 		return urllib2.urlopen(query_url).read().strip()