[BUGFIX beta] Do not use ENV if Ember.ENV exists.

This allows the use of `ENV` in your application without having Ember hijack it for its uses.

To avoid using `window.ENV`, but still provide customization to Ember's boot process you could use something like the following:

```javascript
ENV = 'DO NOT USE ME!!!!';

EmberENV = {DISABLE_RANGE_API: true};
```

I did an audit of all usage of `window.ENV`, and this was the only instance (other than internal tests) where we were always using `window.ENV`.

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The changes to `metamorph` are from tomhuda/metamorph.js#29 (which has been merged).