Update or remove es6-promise dependency

[justinhoward]

Currently es6-promise is listed as a dependency and restricted to ^3.1.2 in package.json. That prevents updating to version 4.0.0 or higher. I see two solutions:

• Update the dependency to >=3.1.2 <5.0.0 to allow the newest version
• Remove the dependency on es6-promise and rely on developers to choose their own polyfill if necessary.

The first is easiest and that might be a good quick fix. I'm guessing that the long-term solution is to remove the dependency and polyfill altogether. This would allow developers to choose their own polyfill or no polyfill at all.

[Auspicus]

I'm ok with us just updating the dependency for now. Do you want to get a PR up with that change? @justinhoward