add yajl dependency

Background:
User tries to save invalid JSON

Error:
```bash
$ EDITOR=vim knife solo data bag edit apps app_1 
ERROR: knife encountered an unexpected error
This may be a bug in the 'solo data bag edit' knife command or plugin
Please collect the output of this command with the `-VV` option before filing a bug report.
Exception: NameError: uninitialized constant Yajl
```