Kill the simulator after completing rake tasks that use it.

I had run into an issue a few times where Cedar was using the colorized reporter even when running from Xcode, which results in pretty poor output. I think this is the same issue as [#64642816]. It turns out that this happens because the simulator process lives on after rake finishes, and it keeps any environment state that it may have had when it was launched, including CEDAR_REPORTER_CLASS.