Fix function getScrollbarWidth

$(document.body).height() gets the same height as $(window).height() in some cases (for example if you use the foundation framework, because it sets the body height to 100%).
Change $(document.body).height() to $(document).height() works fine.

src/remodal.js

@@ -202,7 +202,7 @@
    * @returns {Number}
    */
   function getScrollbarWidth() {
-    if ($(document.body).height() <= $(window).height()) {
+    if ($(document).height() <= $(window).height()) {
       return 0;
     }