Create symbol from selected layers not working
Selected layers are artboards but I get the following message:

![image](https://user-images.githubusercontent.com/9990026/75020764-a433b800-5493-11ea-9d9a-a7b0e9ad3a16.png)

The check for `MSArtboardGroup` here
https://github.com/Ashung/Automate-Sketch/blob/master/automate-sketch.sketchplugin/Contents/Sketch/Symbol/Create_Symbols_from_Selected_Layers.js#L21 Is probably not valid anymore

Maybe the new API can help here:
https://developer.sketch.com/reference/api/#symbol-master 
Not 100% why it is not working.
