[Get rid of / Patch] mailgun-js

[peterpro]

It's outdated & referencing to broken dependencies (broken in Node >=10), see mailgun/mailgun-js-boland#246 with root problem in TooTallNate/node-agent-base#35

I'm very grateful for your cool library, but I'm forced to used https://github.com/mailgun/mailgun-js. It's old as God himself, but not breaking my app :(

[vanntile]

I want to add a heads up to this issue. What would be the minimum necessary to patch this?

[bradenmacdonald]

https://www.npmjs.com/package/mailgun.js is the official replacement