should fix issue #7520, test_nanargmin fails with datetime64 objects

I think this will fix the issue #7520.  The issue appears to be that older versions of ``numpy``, probably less than ``1.7.0``, can't handle ``datetime64`` dtypes very well.  I have added simple check to see if ``numpy`` can handle a standard operation with ``datetime64`` dtypes, and if not it skips those tests.

I can't test this personally since I don't have the older version of ``numpy``.