Install ruby on rails in Ubuntu 12.04

参考：http://ruby-china.org/wiki/install-rails-on-ubuntu-12-04-server
简述如下
1. 更改源并update
2. Install 第三方库
3. Install RVM
4. Install Ruby
5. Install Rails

安装版本
rvm -v

rvm 1.22.9 (master) by Wayne E. Seguin <wayneeseguin@gmail.com>, Michal Papis <mpapis@gmail.com> [https://rvm.io/]
ruby -v

ruby 2.0.0p247 (2013-06-27 revision 41674) [i686-linux]
rails -v
Rails 4.0.0
SQLite3
1.3.8

碰到的问题：
Q1: There is no checksum for 'http://production.cf.rubygems.org/rubygems/rubygems-2.1.3.tgz' or 'rubygems-2.1.3.tgz', it's not possiable to validate it.
A1: rvm get stable or rvm get head
Reference: http://stackoverflow.com/questions/15378312/rvm-install-error-there-is-no-checksum-its-not-possible-to-validate-it