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

Type of change

What type of change does your code introduce to the provider? Please put an x (w/o heading/trailing white spaces) in the boxes that apply:

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

Checklist

Please put an x (w/o heading/trailing white spaces) in the boxes that apply:

• [ ] 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
• [ ] I have added/updated necessary documentation (if appropriate)
• [ ] I have added the following info to the title of the PR (pick the appropriate option for the type of change). This is important because the release notes will include this information.
  • [ ] Feature Request: PRs related to feature requests should have in the title [FeatureRequest: Issue #X] <PR Title>
  • [ ] Bug Fixes: PRs related to bug fixes should have in the title [BugFix: Issue #X] <PR Title>
  • [ ] Tech Debt: PRs related to technical debt should have in the title [TechDebt: Issue #X] <PR Title>
  • [ ] New Release: PRs related to a new release should have in the title [NewRelease] vX.Y.Z

Checklist for Admins

• [x] Label is populated
• [x] PR is assigned to the corresponding project
• [x] PR has at least 1 reviewer and 1 assignee