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)

lib/Voice/VoiceConnection.js

@@ -99,7 +99,7 @@ var VoiceConnection = (function (_EventEmitter) {
 		this.client.internal.sendWS({
 			op: 4,
 			d: {
-				guild_id: null,
+				guild_id: this.server.id,
 				channel_id: null,
 				self_mute: true,
 				self_deaf: false