Rake task fails when you set `config.action_controller.asset_host`

[ProGM]

When you set asset_host in your configs the rake task fails:

config.action_controller.asset_host = 'http://www.some.host.com'

$ rake critical_path_css:generate
Unable to open file '/myapp/publichttp://www.some.host.com/assets/application-d27ba3f146352a973ffa7b384336c02eeba72a60aa8ceeac1d9155987c778b5c.css'

I think it could be easily fixed by changing this line:
https://github.com/mudbugmedia/critical-path-css-rails/blob/master/lib/critical_path_css/configuration.rb#L35

with this:

@manifest_path ||= ActionController::Base.helpers.stylesheet_path(manifest_name, host: '')

If you want, I can create a PR.