Rack incompatibility with rails 4.0.0.rc1

[kcurtin]

I'm not able to install middleman-syntax w/ rails 4.0.0.rc1 - I get the follow error when running bundle install:

Bundler could not find compatible versions for gem "rack":
  In Gemfile:
    middleman-syntax (>= 0) ruby depends on
      rack (~> 1.4.1) ruby

    rails (= 4.0.0.rc1) ruby depends on
      rack (1.5.2)

My Gemfile:

source 'https://rubygems.org'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.0.0.rc1'
gem 'puma'
gem 'pg'

gem 'rspec-rails'

gem 'turbolinks'
gem 'jbuilder', '~> 1.0.1'

gem 'sass-rails', '~> 4.0.0.rc1'
gem 'uglifier', '>= 1.3.0'
gem 'coffee-rails', '~> 4.0.0'
gem 'haml-rails'
gem 'jquery-rails'
gem 'flatui-rails', git: "git://github.com/kcurtin/flatui-rails.git"
gem 'truncate_html'

gem 'redcarpet'
# gem 'pygments.rb'
gem 'middleman-syntax'
gem 'reverse_markdown'

group :doc do
  # bundle exec rake doc:rails generates the API under doc/api.
  gem 'sdoc', require: false
end

group :development, :test do
    gem 'rspec-rails'
    gem 'debugger'
end

[bhollis]

middleman-syntax is a plugin for middleman, not for Rails.