COMPAT: openpyxl deprecation warnings
other issues about warnings
xref #14519 
xref #14539 

tons of warnings on openpyxl 2.4.0 (this is under 3.5), but we have the latest on lots of travis builds.

so need to fix these based on the version, IOW. have a function that introspects the version then does a) if < 2.4.0 (or whatever version triggers these), else does the the new thing.

```
(pandas) bash-3.2$ nosetests pandas/io/tests/test_excel.py  -v
test_ExcelWriter_dispatch (pandas.io.tests.test_excel.ExcelWriterEngineTests) ... /Users/jreback/miniconda3/envs/pandas/lib/python3.5/site-packages/xlwt/ExcelFormulaLexer.py:53: DeprecationWarning: LOCALE flag with a str pattern is deprecated. Will be an error in 3.6
  VERBOSE+LOCALE+IGNORECASE)
ok
test_register_writer (pandas.io.tests.test_excel.ExcelWriterEngineTests) ... ok
SKIP: openpyxl 2.4.0 >= 2.2
test_basics_with_nan (pandas.io.tests.test_excel.Openpyxl22Tests) ... /Users/jreback/miniconda3/envs/pandas/lib/python3.5/site-packages/openpyxl/workbook/workbook.py:179: DeprecationWarning: Call to deprecated function or class remove_sheet (Use wb.remove(worksheet) or del wb[sheetname]).
  def remove_sheet(self, worksheet):
/Users/jreback/miniconda3/envs/pandas/lib/python3.5/site-packages/openpyxl/workbook/workbook.py:179: DeprecationWarning: Call to deprecated function or class remove_sheet (Use wb.remove(worksheet) or del wb[sheetname]).
  def remove_sheet(self, worksheet):
/Users/jreback/miniconda3/envs/pandas/lib/python3.5/site-packages/openpyxl/workbook/workbook.py:179: DeprecationWarning: Call to deprecated function or class remove_sheet (Use wb.remove(worksheet) or del wb[sheetname]).
  def remove_sheet(self, worksheet):
/Users/jreback/miniconda3/envs/pandas/lib/python3.5/site-packages/openpyxl/workbook/workbook.py:179: DeprecationWarning: Call to deprecated function or class remove_sheet (Use wb.remove(worksheet) or del wb[sheetname]).
  def remove_sheet(self, worksheet):
ok
test_bool_types (pandas.io.tests.test_excel.Openpyxl22Tests) ... /Users/jreback/miniconda3/envs/pandas/lib/python3.5/site-packages/openpyxl/workbook/workbook.py:179: DeprecationWarning: Call to deprecated function or class remove_sheet (Use wb.remove(worksheet) or del wb[sheetname]).
  def remove_sheet(self, worksheet):
/Users/jreback/miniconda3/envs/pandas/lib/python3.5/site-packages/openpyxl/workbook/workbook.py:179: DeprecationWarning: Call to deprecated function or class remove_sheet (Use wb.remove(worksheet) or del wb[sheetname]).
  def remove_sheet(self, worksheet):
ok
test_bytes_io (pandas.io.tests.test_excel.Openpyxl22Tests) ... /Users/jreback/miniconda3/envs/pandas/lib/python3.5/site-packages/openpyxl/workbook/workbook.py:179: DeprecationWarning: Call to deprecated function or class remove_sheet (Use wb.remove(worksheet) or del wb[sheetname]).
  def remove_sheet(self, worksheet):
ok
```