mac brew install指定版本

前提，安装brew
第一步：brew update
第二步(例子)：

1. Find the library you're looking for at :
   https://github.com/mxcl/homebrew/tree/master/Library/Formula
2. Click it: https://github.com/mxcl/homebrew/blob/master/Library/Formula/postgresql.rb
3. Click the "history" button to look at old commits: https://github.com/mxcl/homebrew/commits/master/Library/Formula/postgresql.rb
4. Click the one you want: "postgresql: update version to 8.4.4"
5. Click the "raw" button: https://raw.github.com/mxcl/homebrew/fa992c6a82eebdc4cc36a0c0d2837f4c02f3f422/Library/Formula/postgresql.rb
6. Input command：
   brew install https://raw.github.com/mxcl/homebrew/fa992c6a82eebdc4cc36a0c0d2837f4c02f3f422/Library/Formula/postgresql.rb