nsqadmin: update gulpfile for Gulp 4
nsqadmin: update gulpfile for Gulp 4

jehiah/nsq,jehiah/nsq,rpau/nsq,bitly/nsq,bitly/nsq,mreiferson/nsq,bitly/nsq,ploxiln/nsq,mreiferson/nsq,rpau/nsq,rpau/nsq,rpau/nsq,ploxiln/nsq,jehiah/nsq,jehiah/nsq,bitly/nsq,ploxiln/nsq,mreiferson/nsq,ploxiln/nsq,nsqio/nsq,nsqio/nsq,mreiferson/nsq,nsqio/nsq,bitly/nsq,rpau/nsq,ploxiln/nsq,nsqio/nsq,jehiah/nsq,mreiferson/nsq,nsqio/nsq