Restricted Webmock version to fix failing spec

With [WebMock 1.20.3](https://github.com/bblimke/webmock/commit/5554c0d87b6ef341a10c89b344342f1b69621b6d) you can no longer use :with without any parameters, causing this feature to fail
```
cucumber features/evolve/existing_services.feature:35 # Scenario: Stubbing with the contracts
```
I restricted the gem version and added a todo, because I did not find exactly where the empty with was being called.