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](https://github.com/thoughtbot/neat/blob/master/app/assets/stylesheets/settings/_grid.scss) and will need to be updated to just use `modular-scale` if this moves forward.

- [ ] Update Neat to use `modular-scale` instead of `golden-ratio` for its `column` & `gutter` settings
- [ ] Add deprecation note to the [Bourbon documentation](http://bourbon.io/docs/#golden-ratio)
- [ ] Update Neat documentation which mentions `golden-ratio` in the [`column`](http://thoughtbot.github.io/neat-docs/latest/#column) and [`gutter`](http://thoughtbot.github.io/neat-docs/latest/#gutter) sections