Improving the check for NSJSONSerialization.
Improving the check for NSJSONSerialization.

We now only attempt to use NSJSONSerialization if we're compiling
against a Base SDK that includes it.  If we are, we still retain the
NS_CLASS_AVAILABLE-based `class` check to test whether or not it's
available at runtime.

The fallback in both cases is to use JSONKit.

skillz/AFNetworking