Better Message for xlrd Dependencies
Right now if you don't have `xlrd` installed and use `read_excel` without specifying the `engine` keyword you get the following message:

```python-traceback
>>> pd.read_excel("test.xlsx")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/williamayd/.pyenv/versions/3.7.2/lib/python3.7/site-packages/pandas/util/_decorators.py", line 208, in wrapper
    return func(*args, **kwargs)
  File "/Users/williamayd/.pyenv/versions/3.7.2/lib/python3.7/site-packages/pandas/io/excel/_base.py", line 310, in read_excel
    io = ExcelFile(io, engine=engine)
  File "/Users/williamayd/.pyenv/versions/3.7.2/lib/python3.7/site-packages/pandas/io/excel/_base.py", line 819, in __init__
    self._reader = self._engines[engine](self._io)
  File "/Users/williamayd/.pyenv/versions/3.7.2/lib/python3.7/site-packages/pandas/io/excel/_xlrd.py", line 20, in __init__
    import_optional_dependency("xlrd", extra=err_msg)
  File "/Users/williamayd/.pyenv/versions/3.7.2/lib/python3.7/site-packages/pandas/compat/_optional.py", line 93, in import_optional_dependency
    raise ImportError(message.format(name=name, extra=extra)) from None
ImportError: Missing optional dependency 'xlrd'. Install xlrd >= 1.0.0 for Excel support Use pip or conda to install xlrd.
```

This is in spite of the fact that the user may have openpyxl installed and could do something like `pd.read_excel(..., engine="openpyxl")` to get the same code to work

Two issues need to be addressed here:

  - The default `read_excel` call with no `engine` argument should fall back to `openpyxl`, if installed
  - The default error message should direct the user to install openpyxl first and foremost, as xlrd is unmaintained