DEPS: Minimum numpy version for pandas 1.0
There has been some discussion about dropping numpy 1.13, and require 1.15 for pandas 1.0. See https://github.com/pandas-dev/pandas/pull/29212#issuecomment-551856248

I think that'd be nice to reduce our matrix of builds for the CI, other than that I'm not sure what advantages this will have in our code base.

Any opinions @pandas-dev/pandas-core ?