ReferenceError: Unknown plugin "transform-async-to-generator"

### This repo is currently on low maintenance. See README for details

### Problem
Build fails...
```
ERROR in ./scripts/layout/theme.js
Module build failed (from ../node_modules/babel-loader/lib/index.js):
ReferenceError: Unknown plugin "transform-async-to-generator"
```

### Replication steps
```
yarn upgrade --latest @shopify/slate-tools
yarn build
```

