Fix deprecation warning for serve_static_files confi settings

Rails 5 Dropped `serve_static_files` in favor of
`public_file_server.enabled`

DEPRECATION WARNING:

`config.serve_static_files` is deprecated and will be removed in Rails 5.1.

Please use `config.public_file_server.enabled = true` instead.

Author: csexton

spec/dummy/config/environments/production.rb

@@ -20,7 +20,7 @@
   # 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.js_compressor = :uglifier

spec/dummy/config/environments/test.rb

@@ -14,7 +14,7 @@
   config.eager_load = false
 
   # Configure static asset server for tests with Cache-Control for performance.
-  config.serve_static_files = true
+  config.public_file_server.enabled = true
   config.static_cache_control = "public, max-age=3600"
 
   # Show full error reports and disable caching.