inclusion of es6-promise for older browsers compatibility

[tomash]

Hello. Thanks for wrapping up fetch polyfill in a Rails gem, great work!
Is there a chance you could include (or be open to pull request that adds) es6-promise [1] polyfill, so that Promise (and therefore this fetch polyfill) is available on all browsers (including IE and Opera Mini, see caniuse [2])?

[1] https://github.com/jakearchibald/es6-promise
[2] http://caniuse.com/#feat=promises

[vipulnsward]

Woah, apologies I missed this. If you are up for this, I would love to have this included!