add api: setFlushBatchSize

[zihejia]

• add a new api: setFlushBatchSize

  /// Set the number of events sent in a single network request to the Mixpanel server.
  /// By configuring this value, you can optimize network usage and manage the frequency of communication between the client
  /// and the server. The maximum size is 50; any value over 50 will default to 50.
  /// * [flushBatchSize] an int representing the number of events sent in a single network request.
  void setFlushBatchSize(int flushBatchSize) {
    _channel.invokeMethod<void>('setFlushBatchSize',
        <String, dynamic>{'flushBatchSize': flushBatchSize});
  }

• bump iOS to '4.2.0'

[jaredmixpanel]

Nice. LGTM.