Replace multi_json with json

Since [Rails 4 will require Ruby 1.9](https://github.com/rails/rails/commit/e883c06a4f924cc4ba74efe4dad36394fad26fa0) and since Ruby 1.9 includes `json` in the standard library, `multi_json` is no longer necessary. I am a longtime maintainer of `multi_json` but [will stop supporting the library in June](https://github.com/intridea/multi_json/pull/113#issuecomment-17668823).

This patch replaces `multi_json` with stdlib `json`. It will use the `json` gem if a newer version is available.

I have already submitted similar pull requests to [`execjs`](https://github.com/sstephenson/execjs/pull/126), [`sprockets`](https://github.com/sstephenson/sprockets/pull/440), and [`uglifier`](https://github.com/lautis/uglifier/pull/51), so `multi_json` need not be a dependency of Rails 4 applications. This should have the effect of making JSON parsing and generation somewhat faster by removing an abstraction layer that is no longer necessary.