Updated brew.rb for Yosemite (10.10)

```
/usr/local/bin/brew: /usr/local/Library/brew.rb: /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby: bad interpreter: No such file or directory
/usr/local/bin/brew: line 23: /usr/local/Library/brew.rb: Undefined error: 0
```
Mac OS X 10.10 (Yosemite) no longer ships with Ruby 1.8 (only 2.0). I have changed the brew.rb file to use the 'Current' directory found in the Ruby.Framework folder. Unless there is a specific reason to use 1.8, this should prevent the issue above when doing a OS upgrade.