Wrong playback speed indication on Lockscreen

[squatchus]

As I understand this repo is not maintained anymore, but just in case. There is a little typo: https://github.com/appunite/AUMediaPlayer/blob/master/Pod/Classes/AUMediaPlayer.m#L713 which cause wrong playback indication on a lock screen. And it's really easy to fix, by replacing 'MPMediaItemPropertyRating' with 'MPNowPlayingInfoPropertyPlaybackRate' :)