Fix bug where `let` doesn't work in runInContext in old Node versions.
Fix bug where `let` doesn't work in runInContext in old Node versions.

See: https://github.com/nodejs/node/issues/2245

Test Plan:
I switched my Node version back to v4 and confirmed that I was able to run `render_component.js` without error.

Auditors: emily, csilvers

Khan/react-render-server,Khan/react-render-server,Khan/react-render-server