upgrade(rails 5): no longer use rake to invoke rake tasks, use rails

though I left the case alone where the command would previously been
just `bundle exec rake`, as doing the same doesn't work for rails.

config/environments/production.rb

@@ -15,7 +15,7 @@ Railsgoat::Application.configure do
   # config.action_dispatch.rack_cache = true
 
   # Disable Rails's static asset server (Apache or nginx will already do this).
-  config.serve_static_files = false
+  config.public_file_server.enabled = false
 
   # Compress JavaScripts and CSS
   config.assets.compress = true