Require numexpr 2.6.2
 - [x] closes #15213
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

This addresses #15213.  Numexpr 2.6.2 has [wheels for all major platforms already](https://pypi.python.org/pypi/numexpr), and although conda is still at 2.6.1, the [fixes in 2.6.2](https://github.com/pydata/numexpr/blob/master/RELEASE_NOTES.rst#changes-from-261-to-262) makes it strongly recommended for everyone.