Update Ember Model for Ember 3.0

[GavinJoyce]

Upgrade and release 2.18:

• [x] upgrade to Ember 2.18
• [x] verify on Intercom app
• [x] release ember-model 2.18
  • [x] write release notes which point out breaking changes
  • [x] push to npm (we might need to get access to do this)

Upgrade and release 3.0:

• [x] replace bower with npm /cc @patocallaghan
• [x] Upgrade and fix any failing tests
• [x] try out build on intercom app
• [x] release ember-model 3.0

[GavinJoyce]

We'll need to find another way of accessing the CP meta:

https://github.com/ebryn/ember-model/blob/868550cc4c61c2364041ac4652555bb539ece1a9/packages/ember-model/lib/model.js#L135

[GavinJoyce]

https://github.com/ebryn/ember-model/releases/tag/3.0.0