Switching to multijson for json parsing.

We're attempting to parse json in a few of these oneboxes, but Ruby doesn't have a JSON parser in the stdlib like rails gives.

Instead I've opted in to use the incredible https://github.com/intridea/multi_json library which will give our users the ability to pick the fastest or best case json parser. Suggested parser is of course https://github.com/ohler55/oj for its amazing speed.