changed CGrectGetMaxX to CGRectGetMaxY, to get actual header height, not...

there was a bug when using the .swift version - the layout cells would appear far down from the header if the header cell height was set to greater than zero.  i determined it was because CGRectGetMaxX was being used instead of the 'Y' counterpart.  The Obj-C version uses CGRectGetMaxY as well.  This solved the issue on my development app.

Thanks for creating a .swift version!   :)