Fix #1746: Remove dependency on ES2015 code from post-install script
Fix #1746: Remove dependency on ES2015 code from post-install script

This allows Sinon to be installed by legacy node versions.

cjohansen/Sinon.JS,mroderick/Sinon.JS,cjohansen/Sinon.JS,mroderick/Sinon.JS,mroderick/Sinon.JS,fatso83/Sinon.JS,fatso83/Sinon.JS,fatso83/Sinon.JS,cjohansen/Sinon.JS