Log Message:
Update ruby-multi_json to 1.6.0.

1.6.0
-----
* [Add gson.rb support](https://github.com/intridea/multi_json/pull/71)
* [Add MultiJson.default_options](https://github.com/intridea/multi_json/pull/70)
* [Add MultiJson.with_adapter](https://github.com/intridea/multi_json/pull/67)
* [Stringify all possible keys for \ 
ok_json](https://github.com/intridea/multi_json/pull/66)
* [Alias `MultiJson::DecodeError` to \ 
`MultiJson::LoadError`](https://github.com/intridea/multi_json/pull/79)