[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!!!!';

Ember = {ENV: {DISABLE_RANGE_API: true}};
```

I did an audit of all usage of `window.ENV`, and this was the only
instance (other than internal tests), that I could see where we were
using `window.ENV` if `Ember.ENV` was specified.