Moving Ember.K to legacy addon

[thoov]

Apart of emberjs/ember.js#15876

[rwjblue]

Looks good! A few small points:

• Can you call _super in included?
• Add tests like how folks use Ember.K "in the wild"? I'm thinking of something like:

test('asdf', function(assert) {
  let Thing = Ember.Object.extend({
    randomMethod: Ember.K,
  });
  let instance = Thing.create();
  assert.strictEqual(instance.randomMethod(), instance);
});

• Add back the deprecation (e.g. access Ember.K should still issue a deprecation).

[rwjblue]

Also, I just enabled CI here so the next rebase should kick off a build...

[thoov]

@rwjblue Addressed the feedback. Can you retrigger the builds as this is running into the stupid travis-ci build bug right now?