Update APNS App creation example's environment name.
Update APNS App creation example's environment name.

* The APNS environment names in the latest code are "development" and "production", so using "sandbox" will fail with a Bad URI error. Changes the README's example to use "development" so that it'll work with the latest code.

rpush/rpush,mandarin6b0/rpush,amaierhofer/rpush,aried3r/rpush