title: Active admin in Rails 4 throws error "Invalid option key"
body: <p>Rails 4 app working just fine until Active Admin is included (from <a href="https://stackoverflow.com/questions/16426398/active-admin-install-with-rails-4">this thread</a>) in the gemfile.</p>
  
  <p>My Gemfile:</p>
  
  <pre><code>gem 'rails', '4.0.0'
  gem 'rails_12factor', group: :production
  gem 'pg'
  gem 'sass-rails', '~&gt; 4.0.0.rc1'
  gem 'uglifier', '&gt;= 1.3.0'
  gem 'coffee-rails', '~&gt; 4.0.0'
  
  gem 'activeadmin', github: 'gregbell/active_admin'
  
  gem 'protected_attributes'
  gem 'rails-observers'
  gem 'actionpack-page_caching'
  gem 'actionpack-action_caching'
  
  gem 'jquery-rails'
  gem 'turbolinks'
  gem 'jbuilder', '~&gt; 1.2'
  
  group :doc do
    gem 'sdoc', require: false
  end
  </code></pre>
  
  <p>The error is thrown while attempting to start the rails server after a clean bundle install.</p>
  
  <pre><code>/var/lib/gems/1.9.1/gems/actionpack-4.0.0/lib/action_controller/railtie.rb:55:in `block (3 levels) in &lt;class:Railtie&gt;': Invalid option key: page_cache_directory= (RuntimeError)
  </code></pre>
  
  <p>Any help is greatly appreciated!</p>