0.21.0 wheels broken for older (<1.13) numpy version
xref https://github.com/pandas-dev/pandas/issues/16715#issuecomment-347326042

I can reproduce what was reported in the other issue: in a clean env, first installing `python -m pip install numpy==1.11.2` and then `python -m pip install pandas` (0.21.0) results in a broken install, while in principle our wheels should be build against the oldest supported numpy version and this should thus work.

From a quick glance it however seems that the python 2.7 wheels are still build with numpy 1.7: https://github.com/MacPython/pandas-wheels/blob/master/.travis.yml

cc @TomAugspurger @jreback 