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/test.rb

@@ -8,8 +8,8 @@ Railsgoat::Application.configure do
   config.cache_classes = true
 
   # Configure static asset server for tests with Cache-Control for performance.
-  config.serve_static_files = true
-  config.static_cache_control = "public, max-age=3600"
+  config.public_file_server.enabled = true
+  config.public_file_server.headers = { 'Cache-Control' => 'public, max-age=3600' }
 
   # Show full error reports and disable caching.
   config.consider_all_requests_local       = true