The quiet_assets gem used in development (when the assets bundler group is installed) is no longer maintained and so doesn't support Rails 5, because it was merged into sprockets-rails 3.1.0.

sprockets-rails 3.1.0 has a quiet flag that implements the same functionality by wrapping the request in the #silence method on the application logger. Note that the #silence method is only a no-op implementation in the logging gem (https://github.com/TwP/logging/issues/11).