Make coveralls optional in CI builds

[nolar]

What do these changes do?

Fix sporadic issues with coveralls API: either 500s, or 422 Unprocessable Entity — by ignoring them.

Description

It fails often with HTTP 422 Unprocessable Entity when posting results, especially after the retries. Make it optional. E.g. https://travis-ci.org/github/zalando-incubator/kopf/jobs/684865883

This might damage the assumulated coverage data, but failing our builds with no way of retrying (without fake commits) is more troublesome.

Sometimes, it fails with 5xx due to API overload. Retrying would be fine, but then it hits 422.

Issues/PRs

Issues: #99

Type of changes

• Mostly CI/CD automation, contribution experience

Checklist

• The code addresses only the mentioned problem, and this problem only
• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt

[zincr]

🤖 zincr found 0 problems , 0 warnings

✅ Large Commits
✅ Approvals
✅ Specification
✅ Dependency Licensing

[zincr]

🤖 zincr found 1 problem , 0 warnings

❌ Approvals
✅ Large Commits
✅ Specification
✅ Dependency Licensing

Details on how to resolve are provided below

---

Approvals

All proposed changes must be reviewed by project maintainers before they can be merged

Not enough people have approved this pull request - please ensure that 1 additional user, who have not contributed to this pull request approve the changes.

• ✅ Approved by PR author @nolar
• ❌ 1 additional approval needed