Try to use Yajl to workaround json encoding issue

We seem to have this issue too

https://github.com/uken/fluent-plugin-elasticsearch/issues/19

Example:

 temporarily failed to flush the buffer. next_retry=2013-12-18 10:35:12
 +0200 error_class="JSON::GeneratorError" error="source sequence is
 illegal/malformed utf-8" instance=22280300