Replace use of NSData API deprecated in iOS 8

-[NSData dataWithContentsOfMappedFile:] was deprecated in iOS 8.
-[NSData dataWithContentsOfURL:options:error] with
NSDataReadingMappedIfSafe has been around since iOS 5, so this is a
backward compatible change.

I used `NSDataReadingMappedAlways` instead of `NSDataReadingMappedIfSafe` so that the behavior is the same as the deprecated method.