Fix extra call to region empty. Fix #1920

Fix extra call to region empty by unbinding from view 'destroy' before
destroying to view

Add test cases for region.empty only being called once
Correct the placement of the swapOut trigger to be in accordance with the documentation_.

Conflicts:
	src/marionette.region.js
Added "v" before version in UrlRoot property

The new documentation expects the version segment to be prepended with `v` otherwise it will go to a `404` page.
Bump and build v2.2.2