OSX 10.10 uses Ruby 2.0

It's never as simple as it seems, but this got things running on *my* Yosemite machine. One major problem we'll have is the "Homebrew requires Yosemite" problem; since 1.8 was hardcoded in a lot of places, we'll break everyone now that 10.10 uses 2.0.

__Don't accept this PR__. (This PR will help people who may want to point at my branch, when coming to Homebrew to look for an existing issue.) Instead, let's talk about how to fix it. 

Maybe there's a way to:
* Change the version in the install script by looking for the system ruby and replacing it, or
* Calling `#! /usr/bin/env ruby` instead of the full system path (I don't have the historical context to know why that isn't happening anyway)