tests: fix rails app config deprecation

test/dummy_rails/config/environments/production.rb

@@ -20,7 +20,13 @@ Dummy::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_assets = false
+  if config.respond_to?(:serve_static_files)
+    # rails >= 4.2
+    config.serve_static_files = true
+  elsif config.respond_to?(:serve_static_assets)
+    # rails < 4.2
+    config.serve_static_assets = true
+  end
 
   # Compress JavaScripts and CSS.
   config.assets.js_compressor = :uglifier