Update addon to Ember CLI 3.12

[patocallaghan]

This is a follow on to my PR at #653. It appears CI is failing due to a node 6 engine incompatibility issue.

Seeing as we're currently in a 4.x beta period it's probably worth upgrading the addon to ember-cli 3.12 which also drops Node 6 support.

[patocallaghan]

A single test fails in CI: Integration | Validations | Factory - General: call super in validations class with no super property

The error being:

Error: Assertion Failed: An EmberObject based class, (unknown), was not instantiated correctly. You may have either used new instead of .create(), or not passed arguments to your call to super in the constructor: super(...arguments). If you are trying to use new, consider using native classes without extending from EmberObject.

This failing test is unrelated to this PR as this has been failing since 3.11. There's an already an issue for it in #642

[offirgolan]

Closing in favor of #668.