Merge pull request #179 from benny-medflyt/fix-bluebird-warning

Fix warning when using "bluebird"

Author: James Butler

lib/Pool.js

@@ -282,6 +282,7 @@ class Pool extends EventEmitter {
     this._trackOperation(wrappedFactoryPromise, this._factoryCreateOperations)
     .then((resource) => {
       this._handleNewResource(resource)
+      return null
     })
     .catch((reason) => {
       this.emit(FACTORY_CREATE_ERROR, reason)