I have a Rails 4 app that works fine locally but that fails at the precompile assets stage when I try to push to Heroku. The error says `cannot load such file -- rack/cache`. I have tried adding the gem "rack-cache" to my Gemfile, which has no effect. 

Below is an extract of the console whilst pushing. 

```
-----> Removing .DS_Store files
-----> Ruby/Rails app detected
-----> Using Ruby version: ruby-2.0.0
-----> Installing dependencies using Bundler version 1.3.2
       Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin --deployment
       Fetching gem metadata from https://rubygems.org/........
       Fetching gem metadata from https://rubygems.org/..
       Fetching git://github.com/grosser/random_records.git
       Installing rake (10.1.0)
       Installing i18n (0.6.4)
       Installing minitest (4.7.5)
       Installing multi_json (1.7.7)
       Installing atomic (1.1.10)
       Installing thread_safe (0.1.0)
       Installing tzinfo (0.3.37)
       Installing activesupport (4.0.0)
       Installing builder (3.1.4)
       Installing erubis (2.7.0)
       Installing rack (1.5.2)
       Installing rack-test (0.6.2)
       Installing actionpack (4.0.0)
       Installing mime-types (1.23)
       Installing polyglot (0.3.3)
       Installing treetop (1.4.14)
       Installing mail (2.5.4)
       Installing actionmailer (4.0.0)
       Installing actionpack-action_caching (1.0.0)
       Installing activemodel (4.0.0)
       Installing activerecord-deprecated_finders (1.0.3)
       Installing arel (4.0.0)
       Installing activerecord (4.0.0)
       Installing bcrypt-ruby (3.0.1)
       Installing sass (3.2.9)
       Installing bootstrap-sass (2.3.2.0)
       Installing carrierwave (0.8.0)
       Installing climate_control (0.0.3)
       Installing cocaine (0.5.1)
       Installing coffee-script-source (1.6.2)
       Installing execjs (1.4.0)
       Installing coffee-script (2.2.0)
       Installing thor (0.18.1)
       Installing railties (4.0.0)
       Installing coffee-rails (4.0.0)
       Installing dalli (2.6.4)
       Installing orm_adapter (0.4.0)
       Installing warden (1.2.1)
       Installing devise (3.0.0.rc)
       Installing excon (0.6.6)
       Installing fattr (2.2.1)
       Installing formatador (0.2.4)
       Installing json (1.8.0)
       Installing net-ssh (2.6.7)
       Installing net-scp (1.1.1)
       Installing mini_portile (0.5.0)
       Installing nokogiri (1.6.0)
       Installing ruby-hmac (0.4.0)
       Installing fog (0.9.0)
       Installing font-awesome-rails (3.2.1.1)
       Installing gmaps4rails (1.5.6)
       Installing tilt (1.4.1)
       Installing haml (4.0.3)
       Installing highline (1.6.19)
       Installing hike (1.2.3)
       Installing jquery-rails (3.0.1)
       Installing jquery-ui-rails (4.0.3)
       Installing kaminari (0.14.1)
       Installing kgio (2.8.0)
       Installing mysql2 (0.3.12b4)
       Installing nested_form (0.3.2)
       Installing options (2.3.0)
       Installing paperclip (3.4.2)
       Installing pony (1.5)
       Installing pr_geohash (1.0.0)
       Installing progress_bar (1.0.0)
       Installing protected_attributes (1.0.3)
       Installing rack-pjax (0.7.0)
       Using bundler (1.3.2)
       Installing sprockets (2.10.0)
       Installing sprockets-rails (2.0.0)
       Installing rails (4.0.0)
       Installing remotipart (1.0.5)
       Installing safe_yaml (0.9.3)
       Installing sass-rails (4.0.0)
       Installing rails_admin (0.5.0)
       Installing raindrops (0.11.0)
       Using random_records (0.2.0) from git://github.com/grosser/random_records.git (at master)
       Installing rsolr (1.0.9)
       Installing sitemap_generator (4.1.0)
       Installing sunspot (2.0.0)
       Installing sunspot_rails (2.0.0)
       Installing turbolinks (1.2.0)
       Installing uglifier (2.1.1)
       Installing unicorn (4.6.3)
       Your bundle is complete! It was installed into ./vendor/bundle
       Post-install message from haml:
       HEADS UP! Haml 4.0 has many improvements, but also has changes that may break
       your application:
       * Support for Ruby 1.8.6 dropped
       * Support for Rails 2 dropped
       * Sass filter now always outputs <style> tags
       * Data attributes are now hyphenated, not underscored
       * html2haml utility moved to the html2haml gem
       * Textile and Maruku filters moved to the haml-contrib gem
       For more info see:
       http://rubydoc.info/github/haml/haml/file/CHANGELOG.md
       Post-install message from paperclip:
       ##################################################
       #  NOTE FOR UPGRADING FROM PRE-3.0 VERSION       #
       ##################################################
       Paperclip 3.0 introduces a non-backward compatible change in your attachment
       path. This will help to prevent attachment name clashes when you have
       multiple attachments with the same name. If you didn't alter your
       attachment's path and are using Paperclip's default, you'll have to add
       `:path` and `:url` to your `has_attached_file` definition. For example:
       has_attached_file :avatar,
       :path => ":rails_root/public/system/:attachment/:id/:style/:filename",
       :url => "/system/:attachment/:id/:style/:filename"
       Post-install message from sitemap_generator:
       NOTE: SitemapGenerator 4.x uses a new file naming scheme which is more standards-compliant.
       If you're upgrading from 3.x, please see the release note in the README:
       https://github.com/kjvarga/sitemap_generator#important-changes-in-version-4
       The simple answer is that your index file is now called sitemap.xml.gz
       and not sitemap_index.xml.gz, but please take a look and see what else has changed.
       Cleaning up the bundler cache.
-----> Writing config/database.yml to read from DATABASE_URL
-----> Preparing app for Rails asset pipeline
       Running: rake assets:precompile
       rake aborted!
       cannot load such file -- rack/cache Be sure to add rack-cache to your Gemfile
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `block in require'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:374:in `load_rack_cache'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:311:in `block in default_middleware_stack'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:308:in `tap'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:308:in `default_middleware_stack'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/engine.rb:494:in `app'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application/finisher.rb:34:in `block in <module:Finisher>'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:30:in `instance_exec'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:30:in `run'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:55:in `block in run_initializers'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/initializable.rb:54:in `run_initializers'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:215:in `initialize!'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/railtie/configurable.rb:30:in `method_missing'
       /tmp/build_2yeff4uc2n89r/config/environment.rb:7:in `<top (required)>'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `block in require'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:213:in `load_dependency'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/dependencies.rb:228:in `require'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:189:in `require_environment!'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/railties-4.0.0/lib/rails/application.rb:249:in `block in run_tasks_blocks'
       /tmp/build_2yeff4uc2n89r/vendor/bundle/ruby/2.0.0/gems/sprockets-rails-2.0.0/lib/sprockets/rails/task.rb:54:in `block (2 levels) in define'
       Tasks: TOP => environment
       (See full trace by running task with --trace)
 !
 !     Precompiling assets failed.

```