Use chpwd hook instead of overriding cd command

plugins/virtualenvwrapper/virtualenvwrapper.plugin.zsh

@@ -37,10 +37,8 @@ if [[ -f "$wrapsource" ]]; then
       fi
     }
 
-    # New cd function that does the virtualenv magic
-    function cd {
-        builtin cd "$@" && _workon_cwd
-    }
+    # Add hook to chpwd function
+    add-zsh-hook chpwd _workon_cwd
   fi
 else
   print "zsh virtualenvwrapper plugin: Cannot find virtualenvwrapper_lazy.sh. Please install with \`pip install virtualenvwrapper\`."