Works for me :)

nix.rb

require "formula"

# Documentation: https://github.com/Homebrew/homebrew/wiki/Formula-Cookbook
#                /usr/local/Library/Contributions/example-formula.rb
# PLEASE REMOVE ALL GENERATED COMMENTS BEFORE SUBMITTING YOUR PULL REQUEST!

class Nix < Formula
  homepage "http://nixos.org/nix"
  url "http://nixos.org/releases/nix/nix-1.7/nix-1.7.tar.xz"
  version "1.7"
  sha1 "1948671fc60002fdde094beef72ff987333bfbc3"

  depends_on 'bdw-gc'
  depends_on 'sqlite'
  depends_on 'xz'
  depends_on 'libxml2'
  depends_on 'libxslt'
  depends_on 'bison'
  depends_on 'flex'
  depends_on 'pkg-config' => :build
  depends_on 'perl' => :build
  depends_on 'WWW::Curl' => :perl
  depends_on 'DBD::SQLite' => :perl

  def install
    system "./configure", "--prefix=#{prefix}"
    system "gnumake"
    system "gnumake", "install"

    (bin+'nix-init.sh').write <<-EOS.undent
          #!/bin/bash
          source #{prefix}/etc/profile.d/nix.sh
        EOS
    chmod 0775, bin+'nix-init.sh'
  end

  def caveats; <<-EOS.undent
    You can use the 'nix-init.sh' script to prepare your shell environment.

    You need to add the nixpkgs channel:
      
      $ nix-init.sh
      $ nix-channel --add http://nixos.org/channels/nixpkgs-unstable
      $ nix-channel --update

    EOS
  end

  test do
    system "nix-env --help"
  end
end