Region.ensureEl() -> Region._ensureElement()

I was trying to upgrade to v2 of Marionette in a project that uses Backbone.Modal, and my modal views were complaining about `ensureEl()` not being a function any more. Looks like [it's using `_ensureElement()` now](https://github.com/marionettejs/Marionette.Upgrade/blob/14ca4da4/upgrade.py#L161).