Backport switch to `prop-types` package to the v1 branch to add compat with react 16

After updating to react 16, I encountered the following error:

```
TypeError: Cannot read property 'shape' of undefined
./node_modules/react-css-themr/lib/utils/themr-shape.js
node_modules/react-css-themr/lib/utils/themr-shape.js:7
```

Would be great to add a 1.8 release with the switch to the `prop-types` package to maintain compatibility for people like us stuck on toolbox v1 for now.

Start by reviewing the v1 branch and the React 16 failure in node_modules/react-css-themr/lib/utils/themr-shape.js. Confirm that the prop-types package switch is applied and that the v1-compatible 1.8 release resolves the reported error.

Written by the indexing model from the issue text.