API: integrate with openpyxl 2.0-2.1 changes
Builds are failing on Travis due to some issue with Excel (weirdly passed on merge but then started failing a few commits later). Currently investigating. C.f. https://travis-ci.org/pydata/pandas/jobs/35873580

@neirbowj - any idea why these tests stopped passing?

It's all some variation on the following:

```
======================================================================
ERROR: test_tsframe (pandas.io.tests.test_excel.OpenpyxlTests_NoMerge)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.4.1/lib/python3.4/site-packages/pandas/io/tests/test_excel.py", line 562, in test_tsframe
    df.to_excel(path, 'test1')
  File "/home/travis/virtualenv/python3.4.1/lib/python3.4/site-packages/pandas/util/decorators.py", line 88, in wrapper
    return func(*args, **kwargs)
  File "/home/travis/virtualenv/python3.4.1/lib/python3.4/site-packages/pandas/core/frame.py", line 1228, in to_excel
    startrow=startrow, startcol=startcol)
  File "/home/travis/virtualenv/python3.4.1/lib/python3.4/site-packages/pandas/io/excel.py", line 687, in write_cells
    'number_format':{'format_code': self.datetime_format}}))
  File "/home/travis/virtualenv/python3.4.1/lib/python3.4/site-packages/pandas/io/excel.py", line 757, in _convert_to_style_kwargs
    new_v = _conv_to_x(v)
  File "/home/travis/virtualenv/python3.4.1/lib/python3.4/site-packages/pandas/io/excel.py", line 1043, in _convert_to_number_format
    from openpyxl.styles import NumberFormat
ImportError: cannot import name 'NumberFormat'
----------------------------------------------------------------------
```
