Use precmd hook for updating OS X proxy icon

Using the chpwd hook function causes some junk to be printed to STDOUT after
returning from a subshell wherein the working directory was changed. In rare
cases, this can cause issues with 3rd party tools. An example is this issue
with the Python virtualenvwrapper tool:

https://bitbucket.org/dhellmann/virtualenvwrapper/issue/216/lsvirtualenv-and-workon-output-broken-in

plugins/terminalapp/terminalapp.plugin.zsh

@@ -32,7 +32,7 @@ if [[ "$TERM_PROGRAM" == "Apple_Terminal" ]] && [[ -z "$INSIDE_EMACS" ]]; then
     # Register the function so it is called whenever the working
     # directory changes.
     autoload add-zsh-hook
-    add-zsh-hook chpwd update_terminal_cwd
+    add-zsh-hook precmd update_terminal_cwd
 
     # Tell the terminal about the initial directory.
     update_terminal_cwd