[CLEANUP] Remove ember-new-computed dependency

[pangratz]

Since v1.13.0 we use Ember.js-v1.13.0, so this addon is no more needed
since the new computed property syntax is supported out of the box with
this Ember.js version.

[wecc]

I find it weird that this pass :) There's a reference to ember-new-computed in Brocfile.js and computedPolyfill is used in:

• packages/ember-data/lib/system/model/attributes.js
• packages/ember-data/lib/system/relationships/belongs-to.js
• packages/ember-data/lib/system/relationships/has-many.js

[pangratz]

Hmm, good catch. Crazy. Good thing AppVeyor was failing (for a change 😉 ) Haven't seen those. I will clean the references and update the PR...

[wecc]

Thanks!