[BUGFIX beta] Do not use ENV if EmberENV 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};
```
Merge pull request #3947 from rjackson/do_not_use_global_env

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