change nyancat telnet server

miku.acm.uiuc.edu was decommissioned a while ago; this is a sanctioned mirror

plugins/nyan/nyan.plugin.zsh

 if [[ -x `which nc` ]]; then
-  alias nyan='nc -v miku.acm.uiuc.edu 23' # nyan cat
+  alias nyan='nc -v nyan.howes.net.nz 23' # nyan cat
 fi