[BUGFIX beta] Ensure namespaces are cleaned up.

Prior to this change, calls to `Ember.Namespace.byName` would still
return the cached class since it was being saved off to
`Ember.Namespace.NAMESPACES_BY_ID` but not cleared.
Merge pull request #4375 from rjackson/ensure-namespaces-cannot-be-looked-up-after-destroy

[BUGFIX beta] Ensure namespaces are cleaned up.