Plugin fails on Sketch 48
Just upgraded Sketch, and when I run the plugin on some JSON that previously worked, nothing happens.

Dug into the console and found the following error message:

```
TypeError: MSAttributedString.alloc().initWithAttributedString is not a function. (In 'MSAttributedString.alloc().initWithAttributedString(attribStr)', 'MSAttributedString.alloc().initWithAttributedString' is undefined)
line: 767
sourceURL: /Users/mdalgleish/Library/Application Support/com.bohemiancoding.sketch3/Plugins/asketch2sketch.sketchplugin/Contents/Sketch/asketch2sketch.js
column: 72
```