chore: drop Node < 10

[knownasilya]

BREAKING CHANGE: Drop Node < 10

[frontsidejack]

A preview package of this pull request has been published with the tag node-10.
You can try it out by running the following command:

$ npm install @microstates/ember@node-10

or by updating your package.json to:

{
  "@microstates/ember": "node-10"
}

Generated by 🚫 dangerJS against 15cee26

[cowboyd]

excellent idea, cheers!