Deprecate the golden ratio function

The `golden-ratio` function simply calls on the `modular-scale` function and chaining like this can make updating things hard. For example, I have a [PR](https://github.com/thoughtbot/bourbon/pull/440) up to add defaults to the `modular-scale` function, but when I change that, the `golden-ratio` function breaks; so it forces a change there, too.

I think we should just prefer use of the `modular-scale` function. Helps keep things simple and tidy.

**Note**: Neat uses `golden-ratio` to set column and gutter defaults and will need to be updated to just use `modular-scale` if this moves forward.