change from Yajl to FFI_Yajl

Looks like the class of the exception changed in the latest version: 

```bash
ERROR: FFI_Yajl::ParseError: parse error: after key and value, inside map, I expect ',' or '}'
          chef_type": "environment"   "default_attributes": {   },   "
                               (right here) ------^
```

Changed the class of the exception.