Allow http for /ping

I don't think there's a way to configure a single endpoint to be accessible via http when using `config.force_ssl = true`. If it is set to false, you can use `force_ssl` in controllers. There doesn't seem to be an easy way to set a global default and override in individual controllers, so the option is either add force_ssl to every single controller, or exclude a "ping" method for any controller. I opted for the latter since it's less boilerplate code. Let me know if anyone would prefer the former /cc @chcholman @bigeasy