Wrong long int value exported

I have the code:

```python
import tablib

data = tablib.Dataset()
data.headers = ['test_id']

a = 568883383628111872
data.append([a])
with open('test.xls', 'wb') as f:
f.write(data.xls)
```

Which produces one `xls` file with id as `568883383628112000`.