Update voiceConnections on destroy

Instead of manually removing VoiceConnections from
InternalClient.voiceConnections, just integrate the functionality into
VoiceConnection.destroy()

lib/Voice/VoiceConnection.js

@@ -105,6 +105,7 @@ var VoiceConnection = (function (_EventEmitter) {
 				self_deaf: false
 			}
 		});
+		this.client.internal.voiceConnections.remove(this);
 	};
 
 	VoiceConnection.prototype.stopPlaying = function stopPlaying() {