REF: Use numpy percentile instead of scipy.

Code Review on a plane!

While we're revising the percentile code, let's follow [the recommendation in the scipy docs](http://docs.scipy.org/doc/scipy-0.14.0/reference/generated/scipy.stats.scoreatpercentile.html) (see the Note) and use `np.percentile` instead of `scipy.stats.scoreatpercentile`. The former is missing some features in numpy < 1.9 but none that we care about. Its basic functionality goes back before 1.6.

I can't measure any performance difference, in spite of what's promised by the Note. Perhaps the improvements do not apply to our use case. In any case, this removes an import and future-proofs.