openpyxl compatibility warning doesn't catch no openpyxl installed
Just installed 0.14.0 (on Windows 7) and received this warning:

> Installed openpyxl is not supported at this time. Use >=1.6.1 and <2.0.0.

I did not have any version of openpyxl installed though. Perhaps the change made to `pandas/io/excel.py` in #7214 should have been more like:

``` diff
-register_writer(_OpenpyxlWriter)
+
+openpyxl_check = openpyxl_compat.is_compat()
+if openpyxl_check is None:
+    warn('Could not import openpyxl -- not using.')
+elif openpyxl_check:
+    register_writer(_OpenpyxlWriter)
+else:
+    warn('Installed openpyxl is not supported at this time. Use >={} and <{}.'
+            .format(openpyxl_compat.start_ver, openpyxl_compat.stop_ver))
```

---

As a side note, I received the same warning after installing openpyxl 1.9.0 but it was because I didn't have `lxml` (<-- I may have misspelled this module name). 
