Not able to perform consistent sync of Sketch Library File
Hey there, 

to be honest, I'm blown away how easily this turns my html into sketch symbols. Big props for that. 

Let me explain what I'm struggling with:

1. I turn a html file into the `asketch.json` file
1. I trigger the sketch plugin and select both files
1. I save that file
1. I link the saved filed in the preferences as a library
1. I create a new file and use the symbols from the library
1. I update the html and perform step 1-3

I'd expect to have this 
![image](https://user-images.githubusercontent.com/1765584/35919336-0bed4854-0c15-11e8-8127-2f88264f69c0.png)
in my sketch but it does not appear. I'm able to use the updated symbols from the library but **already used symbols** are not updated.


Do you know this issue or  how to fix it? 

thanks for the awesome work <3
