style(rubocop): fix all safe auto correctable offenses

config.ru

@@ -1,5 +1,5 @@
 # This file is used by Rack-based servers to start the application.
 
-require_relative 'config/environment'
-$0="[postal] #{ENV['PROC_NAME']}"
+require_relative "config/environment"
+$0 = "[postal] #{ENV.fetch('PROC_NAME', nil)}"
 run Rails.application