use Yajl instead of to_json

I have found that fluentd force encoding to ASCII-8BIT, as consequence when you call to_json method the encoding fail.

```
2014-02-25 16:52:00 +0900 [warn]: emit transaction failed  error_class=Encoding::UndefinedConversionError error=#<Encoding::UndefinedConversionError: "\xE
6" from ASCII-8BIT to UTF-8>
```

It seems to be resolved by the use Yajl.