[Fix] event listener unsetting on region swap.

### TLDR

Using jquery's `.html` or `.empty` removes event listeners from all nodes that are removed. In the case of marionette this is problematic in that if you are using `preventDestroy` when swapping views in regions your old view would have its DOM events undelegated, thus forcing you to  rebind your events manually. (this was the bug)

Fixes #2159

`.to.be.defined` is a bit loose for my taste, I'd rather check its an array