Update scroll-up-bar.js

deal with IE11(not sure if other IE version has this problem)
(when you scroll-up with mouse wheel,scroll-down also triggered,cause the top bar hidden)
(in fact,not like chrome,scroll with mouse wheel triggered scroll event serveal times) 
but I find when it happened,"y==lastY"(var in the js),so I simply add "if (y==lastY){return;}",now it work on IE11 right,:-P