Switch to GitHub Actions, add refresh request retry options, and add support for ESA v4

[fenichelar]

v5.3.0

# Changelog

- Switch from Travis to GitHub Actions
- Add support for retrying refresh token request
- Add support for ember-simple-auth@v4
- Improve unit tests
- Improve inline documentation

Fixes:

#294
#300
#302

[fenichelar]

@jpadilla GitHub Actions test results don't show up here yet (I think they will show up after this PR is merged), but you can see the results here: https://github.com/fenichelar/ember-simple-auth-token/actions/runs/1369558379. Ember beta and canary are failing, but the rest pass. We need to migrate to Node v12 for Ember beta and canary to work, but I want to release another Node v10 compatible version first. Let me know what you think.

[jpadilla]

nice, thanks!