Support old mobile safari

[psineur]

Old Mobile Safari (like iOS 9) doesn't support const keyword in strict mode and has no window.fetch either.

Changes:

• Fix unhandled exceptions on import (es5 target + libs for Promise/Dom)
• Add polyfill for window.fetch

Test plan:

• using old iPad Mini with iOS 9.3.2 & webpack dev-server
• load website (universal web/electron react-app) containing electron-ga, observe it's not loading - throws in rendered.bundle due to const keywords
• apply changes, install electron-ga-psineur package
• app should load now
• network dev tools tab should have google-analytics.com/batch when events are sent