Reorder JrJackson before json_gem

There was a pull-req for preferring JrJackson over JSON here https://github.com/intridea/multi_json/commit/af8bd9799a66855f04b3aff1c488485950cec7bf, but subsequently it was reverted with this commit https://github.com/intridea/multi_json/commit/d441ec6673d7660693cbcf79316a79057a6d2a3b

This commit changes both the require order and REQUIREMENT_MAP.