Fix 6840 Check emacsclient version instead of emacs's (#6841)

This is much faster.

plugins/emacs/emacs.plugin.zsh

@@ -10,7 +10,7 @@
 # - Configuration changes made at runtime are applied to all frames.
 
 
-if "$ZSH/tools/require_tool.sh" emacs 24 2>/dev/null ; then
+if "$ZSH/tools/require_tool.sh" emacsclient 24 2>/dev/null ; then
     export EMACS_PLUGIN_LAUNCHER="$ZSH/plugins/emacs/emacsclient.sh"
 
     # set EDITOR if not already defined.