anachronistic/apns — pull request #21 by YujiSuzuki (2014-08-23 17:22:20+00:00)
Title: the key “content-available” is the key that enable a silent notification

It is necessary for a silent notification.

https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ApplePushService.html