Not working rack pagespeed application

.gitignore

*~
public/rack-*

.rvmrc

rvm use --create @page-rackspeed-bug

a.js

alert("hello world");

application.rb

require 'rubygems'
require 'bundler/setup'
require 'sinatra'
require 'rack/pagespeed'
require 'haml'

public_dir = File.join(File.dirname(__FILE__), "public") 
use Rack::PageSpeed, :public => public_dir  do
  store :disk => public_dir
  minify_javascripts
  combine_javascripts :hash => { %w(/a.js /b.js) => "app" }
end

get '/' do
  haml :root
end

b.js

alert("hello world bis");

Gemfile

source :rubygems
gem 'sinatra'
gem 'haml'

# I'm using this because I can't build memcached on my machine
gem 'rack-pagespeed', :git => "git://github.com/mcollina/rack-pagespeed.git"

Gemfile.lock

GIT
  remote: git://github.com/mcollina/rack-pagespeed.git
  revision: f491ac696ff73a843a356d5b46aca2c763d9f20d
  specs:
    rack-pagespeed (1.0.3)
      jsmin (= 1.0.1)
      jsmin (= 1.0.1)
      mime-types (= 1.16)
      nokogiri (= 1.4.4)
      nokogiri (= 1.4.4)
      rack (= 1.2.1)
      rack (= 1.2.1)

GEM
  remote: http://rubygems.org/
  specs:
    haml (3.0.25)
    jsmin (1.0.1)
    mime-types (1.16)
    nokogiri (1.4.4)
    rack (1.2.1)
    sinatra (1.2.1)
      rack (~> 1.1)
      tilt (>= 1.2.2, < 2.0)
    tilt (1.2.2)

PLATFORMS
  ruby

DEPENDENCIES
  haml
  rack-pagespeed!
  sinatra

root.haml

!!!
%html
  %head
    %title Bug Bug!

    %script{:src => "/a.js"}
    %script{:src => "/b.js"}
  %body
    Hello world