0.21.1 Windows wheels broken for older (<1.13) numpy version
`RuntimeError: module compiled against API version 0xb but this version of numpy is 0xa`
(0xb corresponds to 1.13.x and 0xa to 1.10.x-1.12.x)

This happens when doing `import pandas` from a Python 3.6 installation having numpy 1.12.0 installed and pandas 0.21.1. On the README it says "NumPy: 1.7.0 or higher" so this should work, right?

In the [release checklist](https://github.com/pandas-dev/pandas/wiki/Release-Checklist) it says:
> Windows wheels are built automatically by Christoph Gohlke.

But I couldn't find any further information like build scripts to verify how the build is done. 