Disable Rack::Cache

It's enabled by default in Rails, but in our case is unnecessary as
there are already several layers of caching in place.  It's also led to
confusion in places due to thing not being updated / 404's being cached
etc.

config/application.rb

@@ -56,5 +56,8 @@ class Application < Rails::Application
 
     # Configure sensitive parameters which will be filtered from the log file.
     config.filter_parameters += [:password]
+
+    # Disable Rack::Cache.
+    config.action_dispatch.rack_cache = nil
   end
 end