Fix editor spec

This was causing a failure for me on CI, see mention in pull request https://github.com/madebymany/sir-trevor-js/pull/10

I think the `afterEach` can be removed as both instances are reset in the `beforeEach` block. Also, this takes care of resetting the value of `instances` to an empty array. I think it makes sense to leave the rest to the garbage collector.

At the top there's a small indentation change which helps a bit with clarity, but that's more a personal opinion.