Improve mocked responses from API

Summary

The current test cases use the data from the Releases API documentation examples https://docs.gitlab.com/ee/api/releases/. However, there are a few edge cases that are not covered and that have caused the Release CLI to fail while creating releases, #121 (closed) & #124 (closed) are some examples.

We should be able to catch more of these issues by improving or expanding the kinds of responses we can get from the API. An example would be to update the test data more frequently, or at least once for every major GitLab release, so we ensure we are testing against the latest expected responses, especially for different inputs like milestones or released_at.