add support for autojump installed via macports

plugins/autojump/autojump.plugin.zsh

-if [ -f `brew --prefix`/etc/autojump ]; then
+if [ -f /opt/local/etc/profile.d/autojump.sh ]; then
+  . /opt/local/etc/profile.d/autojump.sh
+elif [ -f `brew --prefix`/etc/autojump ]; then
   . `brew --prefix`/etc/autojump
 fi