rails 3.1.0.rc4遇到的若干错误
安装了rails 3.1.0.rc4，启动rails时遇到了javascript的运行时问题，
原因是缺少了javascript运行时环境。运行时环境见execjs的项目主页：
https://github.com/sstephenson/execjs

therubyracer - Google V8 embedded within Ruby
therubyrhino - Mozilla Rhino embedded within JRuby
Johnson - Mozilla SpiderMonkey embedded within Ruby
Mustang - Mustang V8 embedded within Ruby
Node.js
Apple JavaScriptCore - Included with Mac OS X
Mozilla SpiderMonkey
Microsoft Windows Script Host (JScript)
列出了若干javascript运行时环境，至少要安装一种吧。

gem install therubyracer