[BUGFIX beta] Total invalidation of arrayComputed should be synchronous

Currently, a total invalidation of an arrayComputed by a non-array dependency does not immediately invalidate. See http://emberjs.jsbin.com/uLeFezIP/1/edit.

This PR makes the invalidation immediate at the cost of losing some coalescing granted by `Ember.run.once`. Ultimately, I don't think this form of coalescing should be tied to the run loop, but rather deferred and coalesced until a subsequent `get`-type operation.

/cc @hjdivad @mixonic