Do not override "cd" function, use "chpwd" hook instead

This allow the plugin work even w/o using "cd", just by typing
directory's name and hitting Enter

plugins/virtualenvwrapper/virtualenvwrapper.plugin.zsh

@@ -37,10 +37,8 @@ for wrapsource in "/usr/bin/virtualenvwrapper.sh" "/usr/local/bin/virtualenvwrap
         fi
       }
 
-      # New cd function that does the virtualenv magic
+      # Add hook to chpwd function
-      function cd {
+      add-zsh-hook chpwd _workon_cwd
-        builtin cd "$@" && _workon_cwd
-      }
     fi
 
     break