Update .travis.yml

[mustafaergul]

Workaround to use newer Ubuntu distros.

What problem does this Pull Request solve?

Please link to the issue number here (issue will be closed when PR is merged): Closes #349

If you don't have an issue created, please create the issue first describing the problem and then open the PR

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• New feature (change that adds new functionality)
• Bug-fix (change that fixes current functionality)
• Tech debt (enhances the current functionality)
• New release (pumps the version)

Checklist

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• I have read and followed the CONTRIBUTING guidelines
• I have added tests that prove my fix is effective or that my feature works
• I have made sure code compiles correctly and all tests are passing by running make test-all
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Checklist for Admins

• Label is populated
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[dikhan]

Thanks for contributing @mustafaergul , really appreciate the quick turnaround!

Sharing here the issue raised in Travis CI Community forum for the ref: https://travis-ci.community/t/travis-ci-linux-hosts-certificate-store-not-up-to-date/13074/2

I see the build is unblocked on the previusly failing step so we can merge this safely even though one of the unit tests is failing which is also due to the change in the hashicorp releases service. Will be tackling that fix in a separate PR though.