Changed for ... of loops to use 'let' as 'const' breaks in firefox

[brunocangs]

I have been using the package with webpack for react, and it breaks on firefox because it says 'missing = in const declaration'. This seems to fix it

[sindresorhus]

It's up to you to transpile it with Babel if you want to use it in older browsers. You can find a more detailed explanation here: sindresorhus/ama#446

If you use Webpack, check out babel-engine-plugin, which transpiles only the dependencies that needs to be transpiled.