CI: numpy dev started failing

[jreback]

https://travis-ci.org/pandas-dev/pandas/jobs/169382808

IIRC some issues w.r.t. int ** int. unrelated to this PR.

[jreback]

cc @shoyer

[gfyoung]

@jreback : The breaking PR is numpy/numpy #8127.

[shoyer]

I think we should just fix the test. Negative integers to negative powers was already broken in previous versions of NumPy.

The other choice would be to coerce all arguments to floats before taking powers.