Use 'yajl' instead of 'json' to dump a record

Fluentd treats the logs as a binay(ASCII-8BIT) by default
because log collector doesn't know application requirements.
In this case, json library should dump such record correctly.
Yajl can do but JSON can't, so we should use 'yajl' instead of 'json'.

This pull request is related to this issue https://github.com/fluent/fluentd/issues/215