Update JsShrink external library.

Summary: It seems that there was a 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