ccxt api change also need since parameter to be 'undefined' for binan…

…ce (#1268)

Also the since argument must be type of undefined see: https://github.com/ccxt/ccxt/blob/master/js/binance.js#L747

extensions/exchanges/binance/exchange.js

@@ -79,7 +79,7 @@ module.exports = function container (get, set, clear) {
       }
 
       var client = publicClient()
-      client.fetchTrades(joinProduct(opts.product_id), '',undefined, args).then(result => {
+      client.fetchTrades(joinProduct(opts.product_id), undefined, undefined, args).then(result => {
         var trades = result.map(function (trade) {
           return {
             trade_id: trade.id,