Switch to Rspec

So sick of all of MiniTests quirks. Got rid of MiniTest and started using Rspec. It's a little bit more advanced than MiniTest (has more features to learn) but it's still a pretty good test framework.

Run the whole test suite:
```shell
rspec spec
```

Run only the thing you are working on:

```shell
rspec spec/messaging/credentials_spec.rb:17 # Line numbers are OK.
```