configures the null cache store in all environments

This commit allows us to use caching and Rails.cache.
Temporarily we configure the null store in all environments,
but those will probably be memcached in the future.

References #235

config/environments/staging.rb

@@ -55,7 +55,7 @@ Rails.application.configure do
   # config.logger = ActiveSupport::TaggedLogging.new(SyslogLogger.new)
 
   # Use a different cache store in production.
-  # config.cache_store = :mem_cache_store
+  config.cache_store = :null_store
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server.
   # config.action_controller.asset_host = 'http://assets.example.com'