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`.
