Use multi_json, because oj is faster then yajl and better use it, if it available

Use multi_json, because oj is faster then yajl and better use it, if it available.

http://www.ohler.com/oj/#Comparisons

MultiJson will try to find most faster available adapter:

https://github.com/intridea/multi_json/blob/master/lib/multi_json.rb#L34-L41