fix squad metric format
The format of the squad metric was wrong.
This should fix #143 

I tested with
```python3
predictions = [
    {'id': '56be4db0acb8001400a502ec', 'prediction_text': 'Denver Broncos'}
]
references = [
    {'answers': [{'text': 'Denver Broncos'}], 'id': '56be4db0acb8001400a502ec'}
]
```