Fix to be fully responsive on all devices

I found that using $(window).width() created a problem as that line also counted the scrollbar width. That created some differences beetween CSS breakpoints and the plugin JS reponsive function as they had a difference of 16px (the width of the scrollbar), these can be seen in the http://kenwheeler.github.io/slick/ responsive example, if you reduce the browser to 1040px width you can check that it will trigger the 1024px breakpoint.