Update AFAudioRouter.m

Add (id\<AVAudioPlayerDelegate\>) in line 31:
[[AVAudioSession sharedInstance] setDelegate:(id\<AVAudioPlayerDelegate\>)self];

To get rid of warning:
Incompatible pointer types sending 'Class' to parameter of type 'id<AVAudioPlayerDelegate>'
Merge pull request #3 from jameszoh/patch-1

Update AFAudioRouter.m