Change NSCalendarUnitDay to NSDayCalendarUnit

This should be a fix for issue #36 which prevented submission to CocoaPods. 

See here: https://travis-ci.org/CocoaPods/Specs/builds/12498782#L155

It looks like it might be a better option to use `NSDayCalendarUnit` instead of `NSCalendarUnitDay` even though the former is deprecated.