[BUGFIX beta] Ensure that ember-debug uses EmberENV.

This brings `ember-debug` in sync with the way `packages/ember-metal/lib/core.js`
pulls in `Ember.ENV`.

See https://github.com/emberjs/ember.js/pull/3947 for more info.
Merge pull request #4025 from rjackson/ensure_debug_follows_same_env_vars

[BUGFIX beta] Ensure that ember-debug uses EmberENV.