`let` breaks old browsers

[vvaryukhin]

Hi. Is it really necessary to use let here?

react-rails/react_ujs/index.js

Line 106 in dadaa74

let component = this.components[cacheId];

It throws an error
SyntaxError: Expected an identifier but found 'component' instead

upd: Oh i see this duplicate #1016

[BookOfGreg]

I agree, and we should support those old browsers where we can. Can you open a PR to fix?