The only way to get working Homebrew on OS X 10.10 for now (10.10 beta was just released a couple of hours ago) is by using an special fork by @jacknagel available at https://github.com/jacknagel/homebrew/tree/rb2.
This version comments out: abort "MacOS too old, see: https://github.com/mistydemeo/tigerbrew" if macos_version < "10.5"
because strings.