openpyxl depreciation warning
With python 2.7.14, the following code

```df.to_excel('output.xlsx')```

produces the following warning:

`/Users/cychu/Library/Python/2.7/lib/python/site-packages/pandas/io/excel.py:784: DeprecationWarning: Call to deprecated function remove_sheet (Use wb.remove(worksheet) or del wb[sheetname]).
  self.book.remove_sheet(self.book.worksheets[0])`




