Send guild_id on voice destroy for bot accounts to work

(thanks qeled for supplying this fix, as per in the node_discord-js discord chat channel :D)

src/Voice/VoiceConnection.js

@@ -62,7 +62,7 @@ export default class VoiceConnection extends EventEmitter {
 			{
 				op : 4,
 				d : {
-					guild_id : null,
+					guild_id : this.server.id,
 					channel_id : null,
 					self_mute : true,
 					self_deaf : false