MAINT: use numpy.nanvar (requires numpy >= 1.8)
This came up in #10679.

Numpy 1.8 implements `numpy.nanvar`, which can replace `pandas.core.nanops._nanvar` when the minimum requirement for Numpy is bumped to 1.8 . The implementation of `_nanvar` in #10679 is in fact based on the corresponding implementation in Numpy.

I haven't checked whether other nanops can be replaced by a suitable Numpy equivalent.
