fix: make agenda.cancel no longer require a callback

closes #249

Author: rschmukler

lib/agenda.js

@@ -269,9 +269,10 @@ Agenda.prototype.now = function(name, data, cb) {
  *  @caller client code, Agenda.purge(), Job.remove()
  */
 Agenda.prototype.cancel = function(query, cb) {
-  // NF refactored 21/04/2015
   this._collection.deleteMany( query, function( error, result ){
-    cb( error, result && result.result ? result.result.n : undefined );
+    if (cb) {
+      cb( error, result && result.result ? result.result.n : undefined );
+    }
   });
 
 };