Use yajl since it is already a dependency

I noticed that both the `json` gem and the `yajl` gem were being used. I don't think there's a reason to use both. This removes the `json` gem as a development dependency and switches all uses of `JSON` to `Yajl`.
Cool, then:

> I'm not sure if we should switch these runtime dependencies to use Yajl or if they should keep using JSON.

:+1: