fix: whenCurrentJobsFinished should wait for all jobs

See https://github.com/OptimalBits/bull/pull/1542/files#r356810178

lib/queue.js

@@ -1191,7 +1191,7 @@ Queue.prototype.whenCurrentJobsFinished = function() {
     return this.bclient.connect();
   });
 
-  return Promise.all([this.processing[0]]).then(() => {
+  return Promise.all(this.processing).then(() => {
     return forcedReconnection;
   });
 };