use json from the stdlib as MultiJson will be discontinued

As described in this comment: https://github.com/intridea/multi_json/pull/113#issuecomment-17668823 `MultiJson` will be discontinued. As ruby `1.9.2` has a json library inside of stdlib we can use this one.

I did not specify the dependency in the Gemfile, people running with 1.8 could still put the `json` gem as a dependency in their Gemfile to get it working.

Closes #153