External Libraries: Update `underscore` to `1.13.8`.

A full list of changes can be found on GitHub: https://github.com/jashkenas/underscore/compare/1.13.7...1.13.8.

Props jonsurrell.
Fixes #64891.

git-svn-id: https://develop.svn.wordpress.org/trunk@62060 602fd350-edb4-49c9-b593-d223f7449a82