Add availability check for iOS 8 attributes.
Add availability check for iOS 8 attributes.

brittonmobile/SnapKit,duanhjlt/SnapKit,zhaoguohui/SnapKit,zht415/SnapKit,SnapKit/SnapKit,Rdxer/SnapKit,dawangjiaowolaixunshan/SnapKit,brittonmobile/SnapKit,CoderJFCK/SnapKit,fhchina/SnapKit,dawangjiaowolaixunshan/SnapKit,ernestopino/SnapKit,duanhjlt/SnapKit,HackRoy/SnapKit,SnapKit/SnapKit,zhaoguohui/SnapKit,zht415/SnapKit,zilaiyedaren/SnapKit,CoderJFCK/SnapKit,HackRoy/SnapKit,Rdxer/SnapKit,398539445/SnapKit,ernestopino/SnapKit,zilaiyedaren/SnapKit,fhchina/SnapKit,398539445/SnapKit