Re-enable ember-runtime build target

The pre-broccoli build system produced `ember-runtime.js`, which I
depend on to interact with `Ember.Object` based models in node. It seems
it was dropped in the switch to broccoli.

This change reintroduces it.
Publish ember-runtime.js for builds.
Merge pull request #4955 from rjackson/generate-ember-runtime

Generate and publish ember-runtime.js.