Remove Public SRWebSocket.h Import

SocketRocket is a dependency, but not part of the public interface to Pusher. If you build an app only using the OS X framework (as opposed to CocoaPods), you’ll get an error saying “SRWebSocket.h cannot be found”.

Since SRWebSocket.h is already imported in the .m file, we can just declare the protocol there. This may break really old versions of Xcode, but anything using Xcode 5 or 6 should be fine (probably 4.3 or 4.2 as well).