ArrowTypeError in squad metrics
`squad_metric.compute` is giving following error
```
ArrowTypeError: Could not convert [{'text': 'Denver Broncos'}, {'text': 'Denver Broncos'}, {'text': 'Denver Broncos'}] with type list: was not a dict, tuple, or recognized null value for conversion to struct type
```

This is how my predictions and references look like
```
predictions[0]
# {'id': '56be4db0acb8001400a502ec', 'prediction_text': 'Denver Broncos'}
```

```
references[0]
# {'answers': [{'text': 'Denver Broncos'},
  {'text': 'Denver Broncos'},
  {'text': 'Denver Broncos'}],
 'id': '56be4db0acb8001400a502ec'}
```

These are structured as per the `squad_metric.compute` help string.