Use on_exit callback instead of teardown for bank-account tests

The teardown functionality was removed from ExUnit and replaced by the [on_exit function](https://github.com/elixir-lang/elixir/issues/2319). This PR fixes the unit tests so they'll actually run.

That said, I'd also like to note that keeping the `BankAccount.close_bank/1` function in the exercise forces you to use `GenServer.start/3` instead of `GenServer.start_link/3` which would automatically clean things up for you when the parent dies...which is a better/safer way to go.