An alternative solution is to simply add a route to your routes.rb
It basically catches the Apple request and renders a 404 back to the client. This way your log files aren't cluttered.
# routes.rb at the near-end
match '/:png', via: :get, controller: 'application', action: 'apple_touch_not_found', png: /apple-touch-icon.*\.png/
then add a method 'apple_touch_not_found' to your application_controller.rb
# application_controller.rb
def apple_touch_not_found
render plain: 'apple-touch icons not found', status: 404
end