Refactor buildRegion to simplify building from different regionConfig types

Reopening (originally #1602) against v2.1 per @samccone's request in gitter. This fixes #1562.
I would just do:

```js
this.buildRegion = function() {
  Marionette.Region.buildRegion(this.BarRegion, this.DefaultRegionClass);
}.bind(this);
```