Use yajl-ruby for json encoding (PR #771)

Implemented recommendation based on TODO found in the original source.

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hmmm … we already have oj, @nlalonde is there a reason you wanted yajl, oj is faster anyway and we already have a gazzilion gems

At the time, I found some benchmarks that claimed that yajl was faster for json encoding, but I didn’t try it or benchmark it myself. If oj is faster, then we can just stick with that.

Closing in favor of #776 / Use oj for json encoding.