brew install drush 7.0-dev

drush.rb

require File.expand_path("/usr/local/Library/Taps/homebrew/homebrew-php/Requirements/php-meta-requirement", __FILE__)

class Drush < Formula
  desc "A command-line shell and scripting interface for Drupal"
  homepage "https://github.com/drush-ops/drush"
  url "https://github.com/drush-ops/drush/archive/7.x.tar.gz"
  sha1 "6e648c6056a6a6d738ad5665ce842083b22e7e61"
  version "7.0-dev"

  head do
    url "https://github.com/drush-ops/drush.git"
  end

  depends_on PhpMetaRequirement
  depends_on "composer" => :build
  depends_on "php53" if Formula["php53"].linked_keg.exist?
  depends_on "php54" if Formula["php54"].linked_keg.exist?
  depends_on "php55" if Formula["php55"].linked_keg.exist?
  depends_on "php56" if Formula["php56"].linked_keg.exist?

  def install
    system "composer", "install"

    prefix.install_metafiles
    File.delete "drush.bat"
    libexec.install Dir["*"]
    (bin+"drush").write <<-EOS.undent
      #!/bin/sh

      export ETC_PREFIX=${ETC_PREFIX:=#{HOMEBREW_PREFIX}}
      export SHARE_PREFIX=${SHARE_PREFIX:=#{HOMEBREW_PREFIX}}

      exec "#{libexec}/drush" "$@"
    EOS
    bash_completion.install libexec/"drush.complete.sh" => "drush"
  end

end

File should be saved in /usr/local/Library/Formula/drush.rb
Then run brew install drush