Enable VIEW_PRESERVES_CONTEXT warning

`ember-views/lib/views/view.js` uses Ember.VIEW_PRESERVES_CONTEXT
to determine if it should warn, but this line of code squashed
the `warn` setting to true.
Merge pull request #28 from zendesk/chris/enable_view_preserves_context_warning

Fix VIEW_PRESERVES_CONTEXT `warn` setting