I am not sure how you are checking the certificate, but curl is able to verify it.
https://travis-ci.org/github/BanzaiMan/travis_production_test/jobs/696174371#L470
https://travis-ci.org/github/BanzaiMan/travis_production_test/jobs/696174372#L184
https://travis-ci.org/github/BanzaiMan/travis_production_test/jobs/696174373#L230
If you would like, we can enable the debug feature for you, so that you can troubleshoot this further.