Dependencies cache in Wercker

Question

I'm using Wercker on my Ruby on Rails project. My wercker.yml looks like this:

box: ruby
services:
  - postgres
build:
    steps:
        - script:
            name: Nokogiri fix
            code: bundle config build.nokogiri --use-system-libraries
        - bundle-install
        - rails-database-yml
        - script:
            name: Set up db
            code: bundle exec rake db:schema:load RAILS_ENV=test
        - script:
            name: rspec
            code: bundle exec rspec
    after-steps:
        - slack-notifier:
            url: $SLACK_URL
            channel: notifications
            username: werckerbot

The problem is that wercker every time download gems with bundle install instead of get them from cache. With that my build takes almost 2 minutes instead of a few seconds... How can I fix that?