[CHEF-4337] Allow IPv6 addresses in chef_server_url

http://tickets.opscode.com/browse/CHEF-4337

Due to improper usage of URI#host (in lieu of URI#hostname), chef-client was unable to connect to the server using direct IPv6 URI like "http://[::1]/".

See http://bugs.ruby-lang.org/issues/3788 for more details.