Get the version in a way that actually works with Xcode 5

Getting the version number from the plist only works when Realm is built as a dynamic framework, as otherwise our plist doesn't actually end up in the final executable at all. As our release builds are currently built with Xcode 5 and thus not dynamic frameworks, this resulted in Realm reporting that an update was available on every check.

@alazier 
This is a bit confusing - maybe put the @ in the macro? Or leave as cstring and do strcmp?