Release Underscore 1.7.0

Addressing #1798. https://github.com/jashkenas/underscore/compare/1.6.0...15a757dfa5c9cb20161153821c4e86354497011f

This is a fairly major release, dropping most natives support (#1525) and adding `_.iteratee` callbacks everywhere (#1582). Might it make sense for this to be Underscore 2.0?