fix issues in iOS 9

[chenkun24]

[lukescott]

Anyone here interested in maintaining this? I just need someone to accept PR's that fix issues without changing too much. At the moment I don't have time to do so.

In the future when things calm down I plan to write a new flow layout with drag & drop. It's a different approach than the current lib, and will likely be in a different repo.

[chenkun24]

Thanks @wsidell for finding out the problem, and I added codes mentioned by @cms5822.

I find out that header or footer elements are always at the tail of the elements array. So another way to change the supplementary problem is to rewrite the comparator and put supplementary elements at the tail of the elements array, but that would make a big change to the comparator. So I just solve it simply.