Add .rspec file to allow running individual tests

We can now run tests without explicitly requiring the spec_helper
file in them, following the same approach as email-alert-api [1].

Example:

```
bundle exec rspec ./spec/use_cases/slack/send_messages_spec.rb
```

[1]: https://github.com/alphagov/email-alert-api/blob/master/.rspec

.rspec

@@ -0,0 +1 @@
+--require spec_helper