Replace yajl dependency with MultiJSON

Instead of using the json gem, or trying to require yajl, this patch uses Multi Json which takes care of deciding which json engine to use, and includes a vendored json implementation if one is not available (okjson).

Yajl is the preferred option in MultiJSON.

MultiJSON is used in resque. They also replaced direct yajl dependency https://github.com/defunkt/resque/pull/318