Replace multi_json with stdlib json

Ruby 1.9 standard library includes `json`, and `multi_json` will [no longer be maintained as of June](https://github.com/intridea/multi_json/pull/113#issuecomment-17668823). This aligns with [planned stoppage](http://www.ruby-lang.org/en/news/2011/10/06/plans-for-1-8-7/) on ruby 1.8.7.

This patch should also improve JSON parsing performance, and make life more better!