Update config.serve_static_assets to config.serve_static_files (#240)

The configuration option `config.serve_static_assets` has been renamed
to `config.serve_static_files in Rails 4.2 [1], and the old name will
be removed in Rails 5.0 [2].

[1] https://www.github.com/rails/rails/pull/18100
[2] rails/rails@463b5d7

config/environments/production.rb

@@ -33,7 +33,7 @@
 
   # Disable Rails's static asset server
   # In production, Apache or nginx will already do this
-  config.serve_static_assets = true # true so Webrick testing of production mode works
+  config.serve_static_files = true # true so Webrick testing of production mode works
 
   # Enable serving of images, stylesheets, and javascripts from an asset server
   # config.action_controller.asset_host = "http://assets.example.com"

config/environments/test.rb

@@ -34,7 +34,7 @@
   config.active_support.deprecation = :stderr
 
   # Configure static asset server for tests with Cache-Control for performance
-  config.serve_static_assets = true
+  config.serve_static_files = true
   config.static_cache_control = "public, max-age=3600"
 
   # Raise exception on mass assignment protection for Active Record models