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.