update package to include babel-polyfill as dependency

[e-e-e]

I just realised that the es5 compatibility that I added a few months ago is broken for anyone who does not have babel-polyfill in their package.
This is a simple fix to remedy that problem. Sorry for the silly mistake.

[pfrazee]

I appreciate the PR! But I'm not super enthusiastic about including those deps in my projects, since I don't use es5. Is there any way we can do this differently?

[e-e-e]

I completely understand. I was kind of surprised that babel expects such dependency after already transpiling the code - seems poor practice. I did a bit of research on how to avoid it and could not find anything obvious, but let me have another look. Perhaps now node 6 is LTS we could just increase es5 to es6 which I don’t think would require the polyfill for promises, etc.