check if autojump is installed

plugins/autojump/autojump.plugin.zsh

-if [ -f /usr/share/autojump/autojump.zsh ]; then # debian and ubuntu package
-  . /usr/share/autojump/autojump.zsh
-elif [ -f /etc/profile.d/autojump.zsh ]; then # manual installation
-  . /etc/profile.d/autojump.zsh
-elif (`command -v brew >/dev/null`); then # mac os x with brew
-  if [ -f `brew --prefix`/etc/autojump ]; then
+if [ $commands[autojump] ]; then # check if autojump is installed
+  if [ -f /usr/share/autojump/autojump.zsh ]; then # debian and ubuntu package
+    . /usr/share/autojump/autojump.zsh
+  elif [ -f /etc/profile.d/autojump.zsh ]; then # manual installation
+    . /etc/profile.d/autojump.zsh
+  elif [ $commands[brew] -a -f `brew --prefix`/etc/autojump ]; then # mac os x with brew
     . `brew --prefix`/etc/autojump
   fi
 fi