Use set_value_explicit instead of set_explicit_value

set_value_explicit is in openpyxl 1.6, changed in openpyxl 1.8, but
there is code in 1.8 to set set_value_explicit to set_explicit_value for
compatibility

Author: Dr-Irv

pandas/io/excel.py

@@ -711,7 +711,7 @@ def write_cells(self, cells, sheet_name=None, startrow=0, startcol=0):
             new_data_type = xcell.data_type_for_value(cell.val)
             if (isinstance(cell.val, compat.string_types)
                     and new_data_type != xcell.TYPE_STRING):
-                xcell.set_explicit_value(cell.val)
+                xcell.set_value_explicit(cell.val)
             else:
                 xcell.value = _conv_value(cell.val)
             style = None