Fixed numVisibleItems in Scroller component

[psolom]

I got the case when numVisibleItems is calculated as 6 instead of 7 days and that is ruins paged scrollable. I found that before Math.floor() being applied the raw value is 6.94 which, I believe, is ok since it's calculated upon rounded itemWidth & marginHorizontal values. But when you floor the numVisibleItems itself it ends up in wrong days & unexpected paging behavior. This PR suggests to replace Math.floor() with Math.round()

I suppose it also could be a fix for issue #269

[peacechen]

Thanks @psolom 👍

[psolom]

sure, and take a look #279 please
that's the only thing I need to solve before I can use your plugin in my app