don't use ExcelWriter with databook

I've noticed that using ExcelWriter in databook gives an KeyError. Looked around a bit in the openpyxl code and found the comment that ExcelWriter shouldn't be used, so I've changed it.

https://bitbucket.org/ericgazoni/openpyxl/src/638995eb271c/openpyxl/workbook.py
```python
def save(self, filename):
        """Save the current workbook under the given `filename`. 
        Use this function instead of using an `ExcelWriter`.
```