rails: disable Server-Timing header in production.

This produces random `nil can't be coerced into Float (TypeError)`
errors in production because of a Rails bug. This may also be the cause
of random `An unhandled lowlevel error occurred. The application logs
may have details` errors.

https://github.com/rails/rails/issues/44167

config/environments/production.rb

@@ -17,7 +17,9 @@ Rails.application.configure do
   config.action_controller.perform_caching = true
 
   # Enable server timing
-  config.server_timing = true
+  # XXX Re-enable after the fix for rails 44167 is released
+  #config.server_timing = true
+  config.server_timing = false
 
   # Ensures that a master key has been made available in either ENV["RAILS_MASTER_KEY"]
   # or in config/master.key. This key is used to decrypt credentials (and other encrypted files).