Relicense to dual MIT/Apache-2.0

[emberian]

This pull request was automatically generated because my scripts detected that
everyone in the relicensing issue agreed to
relicensing.

License headers in files were not updated, if they exist. I've reproduced an
example license header below if you want to copy-paste it in. If you don't
have license headers, don't feel compelled to add
them.

// Copyright 2016 histogram Developers
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.

I recommend publishing a new patch version (the z in x.y.z) so that the
metadata propagates to crates.io.

Please be sure to check the README.md for style. I've try to
review these PRs as they go out, but I might miss something.

Thanks for relicensing!

(For my bookkeeping: this is repository id histogram-3cef8d2c-d46e-4c4e-b0d1-80615b49ee67)