getWithDefault deprecated on Ember canary

[gitKrystan]

Our Ember canary tests are failing with:

Error: Using getWithDefault has been deprecated. Instead, consider using Ember get and explicitly checking for undefined.

See emberjs/rfcs#554

Looks like getWithDefault is used here in startMirage:

ember-cli-mirage/addon/start-mirage.js

Line 33 in dcef77a

let discoverEmberDataModels = getWithDefault(env['ember-cli-mirage'] || {}, 'discoverEmberDataModels', true);

I can open a PR with a proposed fix.