Support for iOS8 extensions compatibility

Removed reference to `setShouldExecuteAsBackgroundTaskWithExpirationHandler` for extensions compatibility in iOS8 (no such thing as background tasks in extensions so the compiler will balk if this code is included in extensions).

This uses the AF_APP_EXTENSIONS variable already defined by AFNetworking (For reference, see the AFNetworking release notes for version 2.3.0 https://github.com/AFNetworking/AFNetworking/releases) so it will just work without the need to create an ad-hoc variable.