运行提示错误：no such file to load -- watir/ie

require 'rubygems'
require 'watir'
ie = Watir::IE.new.start("http://www.google.cn")
ie.text_field(:name,"q").set("Watir")
ie.button(:name,"btnG").click

求解~~~~~
运行提示错误：no such file to load -- watir/ie

要先安装的：
[code="ruby"]
sudo gem install watir
[/code]