use yajl for json serialization

we've always had yajl in our Gemfile but to_json always fell back to the json gem (which in turn delegates to other implementations, such as ActiveSupport which has known bugs).

Yajl is not having any problems dealing with non-BMP unicode characters, and has the added benefits of being fast* and not requiring any substantial code changes.

(*) http://japgolly.blogspot.com/2012/04/ruby-json-libraries.html

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