如果安装的时候出现：
C:\Users\liyingxuan01>gem install watir
ERROR: Error installing watir:
The 'ffi' native gem requires installed build tools.
Please update your PATH to include build tools or download the DevKit
from 'http://rubyinstaller.org/downloads' and follow the instructions
at 'http://github.com/oneclick/rubyinstaller/wiki/Development-Kit'
解决方案：
1、下载DevKit
DevKit-tdm-32-4.5.2-20110712-1620-sfx.exe
2、解压DevKit
解压到C:\DevKit
3、安装并测试
cd到解压目录下：
C:\DevKit>ruby dk.rb init
[INFO] found RubyInstaller v1.9.3 at C:/Ruby193
Initialization complete! Please review and modify the auto-ge
'config.yml' file to ensure it contains the root directories
of the installed Rubies you want enhanced by the DevKit.
C:\DevKit>ruby dk.rb install
[INFO] Updating convenience notice gem override for 'C:/Ruby1
[INFO] Installing 'C:/Ruby193/lib/ruby/site_ruby/devkit.rb'
C:\DevKit>gem install rdiscount --platform=ruby
Fetching: rdiscount-1.6.8.gem (100%)
Temporarily enhancing PATH to include DevKit...
Building native extensions. This could take a while...
Successfully installed rdiscount-1.6.8
1 gem installed
Installing ri documentation for rdiscount-1.6.8...
Installing RDoc documentation for rdiscount-1.6.8...