Replace Yajl with Oj

[michaeldauria]

I stumbled across this last night: https://github.com/ohler55/oj

I don't consider this to be something we actually pull in, just wanted to put it out there and get some thoughts on it.

[cicloid]

It is faster indeed, but most projects wont notice, the change should be transparent for everyone.

[romanbsd]

Just for the record, in my synthetic tests the Oj was performing a bit better than Yajl, but was using much more memory.