Fix Rails 5 deprecation warning for static_cache_control

DEPRECATION WARNING:

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

Please use

    config.public_file_server.headers = { 'Cache-Control' => 'public, max-age=3600' }

instead.

Author: csexton

spec/dummy/config/environments/test.rb

@@ -15,7 +15,7 @@
 
   # Configure static asset server for tests with Cache-Control for performance.
   config.public_file_server.enabled = true
-  config.static_cache_control = "public, max-age=3600"
+  config.public_file_server.headers = { 'Cache-Control' => 'public, max-age=3600' }
 
   # Show full error reports and disable caching.
   config.consider_all_requests_local       = true