Unbind scroll event when reinstantiated

Scroll trigger would only take effect on first load: navigating back to
the same page would fail to auto-load the next page on scroll.

Solution from: magoosh#24

app/assets/javascripts/jquery.infinite-pages.js.coffee

@@ -147,6 +147,8 @@ Released under the MIT License
       # Have we waited enough time since the last check?
       shouldCheck = -> +new Date > lastCheckAt + scrollDelay
 
+      console.log "Unbinding..."
+      @$context.unbind('scroll')
       @$context.scroll ->
         scrollHandler() if shouldCheck() # Call check once every scrollDelay ms
         if scrollTimeout