ATAppConfigurationUpdateTask needs to retain self before updating task state

[wooster]

This method:

- (void)configurationUpdaterDidFinish:(BOOL)success {
    @synchronized(self) {
        if (configurationUpdater) {
            if (!success) {
                self.failed = YES;
                [self stop];
            } else {
                self.finished = YES;
            }
        }
    }
}

Is unsafe because self.failed may cause the task to be deallocated. This entire statement should be wrapped in a retain/release pair, as with the other tasks.