Update `JsShrink` external library.

Summary: It seems that there was a [[https://github.com/vrana/JsShrink/commit/17cbfacae67dede6d94d94ce92214c8ca31d858e | recent-ish commit]] to the `JsShrink` external library. I'm not sure what this commit actually achieves, but we may as well bring it upstream.

Test Plan: Eyeball it.

Reviewers: vrana, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9414