Improve ES6 compatibility

[randycoulman]

When using this library in a react-native project that uses Jest for testing, I had to ensure that this module gets transpiled by Jest using its transformIgnorePatterns setting.

There are a number of other libraries that have the same problem, but it should be possible to make this library more universally-usable, perhaps using the module key in package.json. I'm not (yet) clear on the details, but I know there are a number of articles and blog posts around about how to do this well.