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.
@FrancescAlted maybe i was not clear on my earlier comment
we are dropping support before 2.4.6

not requiring latest versions 

 Ok.  My first thought was requiring the newest numexpr version, which should be quite spread by the time that pandas reaches 0.20, but I agree that could be too aggresive.  Closing in favor of PR #15383. 