Replace multi_json with json

`multi_json` will [no longer be supported](https://github.com/rails/rails/pull/10576) in June. This replaces it with `json`.

- I went for the current release, because I don't have a compelling reason to use an older release.
- Retain the `pretty: true` we were using in `multi_json`, but using `#pretty_generate` in `json`.

Fixes #101 

/cc @amfeng - if you get a chance!