Use defeatureify to strip debug messages.

Recent changes to `defeatureify` allow stripping of debug messages. This will
allow us to use multi-line debug messages (for example).

To enable this the format of `features.json` has changed.

Before:

```javascript
{
 "foo-feature": true
}
```

After:

```javascript
{
 "features": {
   "foo-feature": true
 },
 "debugStatements": ["Ember.warn", "Ember.assert", "Ember.deprecate",
"Ember.debug", "Ember.Logger.info"]
}
```

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This PR requires changes to be made to Ember's `feature.json` so the integration tests will fail. I have tested locally with a patched version of Ember and all tests pass. Once this is merged I will submit the corresponding PR to Ember to update the `features.json` properly.

----

Requires https://github.com/thomasboyt/defeatureify/pull/8 to be merged and `defeatureify` 0.2.0 to be published.