Ship with rails-html-sanitizer instead.

Should have 4.2.0 label (I don't have access).

cc @chancancode @rafaelfranca 
If we do that I get this error:
```
1) Error:
ApplicationTests::DefaultStackTest#test_the_sanitizer_helper:
NameError: uninitialized constant ActionView::Helpers::AssetUrlHelper
    /Users/kasperhansen/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sprockets-rails-3.0.0.beta1/lib/sprockets/rails/helper.rb:41:in `<module:Helper>'
    /Users/kasperhansen/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sprockets-rails-3.0.0.beta1/lib/sprockets/rails/helper.rb:7:in `<module:Rails>'
    /Users/kasperhansen/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sprockets-rails-3.0.0.beta1/lib/sprockets/rails/helper.rb:6:in `<module:Sprockets>'
    /Users/kasperhansen/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sprockets-rails-3.0.0.beta1/lib/sprockets/rails/helper.rb:5:in `<top (required)>'
    /Users/kasperhansen/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sprockets-rails-3.0.0.beta1/lib/sprockets/rails/environment.rb:2:in `require'
    /Users/kasperhansen/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sprockets-rails-3.0.0.beta1/lib/sprockets/rails/environment.rb:2:in `<top (required)>'
    /Users/kasperhansen/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sprockets-rails-3.0.0.beta1/lib/sprockets/railtie.rb:6:in `require'
    /Users/kasperhansen/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sprockets-rails-3.0.0.beta1/lib/sprockets/railtie.rb:6:in `<top (required)>'
    /Users/kasperhansen/Documents/code/rails/railties/lib/rails/all.rb:13:in `require'
    /Users/kasperhansen/Documents/code/rails/railties/lib/rails/all.rb:13:in `block in <top (required)>'
    /Users/kasperhansen/Documents/code/rails/railties/lib/rails/all.rb:11:in `each'
    /Users/kasperhansen/Documents/code/rails/railties/lib/rails/all.rb:11:in `<top (required)>'
    /private/var/folders/dt/lr2x8xhd5_5bqs56ql7wbt5h0000gn/T/d20140903-12964-1u6mpl3/app/config/boot.rb:2:in `require'
    /private/var/folders/dt/lr2x8xhd5_5bqs56ql7wbt5h0000gn/T/d20140903-12964-1u6mpl3/app/config/boot.rb:2:in `<top (required)>'
    /private/var/folders/dt/lr2x8xhd5_5bqs56ql7wbt5h0000gn/T/d20140903-12964-1u6mpl3/app/config/application.rb:1:in `require'
    /private/var/folders/dt/lr2x8xhd5_5bqs56ql7wbt5h0000gn/T/d20140903-12964-1u6mpl3/app/config/application.rb:1:in `<top (required)>'
    /private/var/folders/dt/lr2x8xhd5_5bqs56ql7wbt5h0000gn/T/d20140903-12964-1u6mpl3/app/config/environment.rb:2:in `require'
    /private/var/folders/dt/lr2x8xhd5_5bqs56ql7wbt5h0000gn/T/d20140903-12964-1u6mpl3/app/config/environment.rb:2:in `<top (required)>'
    test/application/default_stack_test.rb:36:in `require'
    test/application/default_stack_test.rb:36:in `block in <class:DefaultStackTest>'
```

Requiring it after the environment makes the assertion fail like so:
```
1) Failure:
ApplicationTests::DefaultStackTest#test_the_sanitizer_helper [test/application/default_stack_test.rb:39]:
Expected: "Rails::DeprecatedSanitizer"
  Actual: "Rails::Html::Sanitizer"
```