How to upload test reports of Kotlin sources to Coveralls?

[edit August 2020] @nbaztec wrote a plugin to support Kotlin, please see his answer.

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Old answer:

Kotlin is not supported by Coveralls, see for example this open isse that was mentioned in the question as well (in the question it was also mentioned that the workaround presented there does not work): https://github.com/kt3k/coveralls-gradle-plugin/issues/77

Solution: try Codecov.io instead. Install it to GitHub using the Marketplace and add to your .travis.yml

after_success:
  - bash <(curl -s https://codecov.io/bash)

Then commit and push, done!

You can view the result (after the build finished) at https://codecov.io/gh/githubaccountname/reponame