Ignore non-array jsonProps

fix #23
fix brunch/brunch#841

This is the backward-compatible approach. Completely dropping support for defining dependency files under `scripts` and `styles` is also viable, especially considering that defining `main` is a requirement anyway.
This line could be refactored several ways, not sure which style should prevail here. An alternate I also like is

```js
list = list.concat(Array.isArray(pkg[property]) ? pkg[property] : []);
```

or of course it could also stop being a one-liner if both of these alternatives are too convoluted