Pact specs

See https://github.com/flapjack/flapjack-diner/pull/35

At the moment I'm copying in the generated JSON pact file from `flapjack-diner' using

```
bundle exec rspec spec && cp spec/pacts/flapjack-diner-flapjack.json ../flapjack/spec/service_consumers/pacts/flapjack-diner_v1.0.json
```

but, per https://github.com/realestate-com-au/pact/wiki/Sharing-pacts-between-consumer-and-provider , it might be good to get that looking elsewhere when, e.g., building in CI. (I don't like adding network dependencies when developing directly.)