Act as a polyfill about ember-get-helper

[rwjblue]

Yeah, I believe I was a tad over-zealous when adding that warning in #9. I'm perfectly happy to have it removed when ember-get-helper is included by an addon, but I think the warning is still good for apps that have moved to 2.0.0.

My suggestion (needs to be tested with addon and apps) would be:

  treeFor: function() {
    if (this._checkerForEmber.lt('2.0.0-beta.1')) {
      return this._super.treeFor.apply(this, arguments);
    } else if (this.parent === this.project) { 
      // this means that we are being included directly in an app
      // and not as a nested addon
      console.warn('ember-get-helper is not required for Ember 2.0.0 and later (a default `get` helper is included with Ember), please remove from your `package.json`.');
    }
  }