Fix jdk11 issue for travis

[htynkn]

What is the purpose of the change

Travis build fail for openjdk11 for all PR which open in recent few days.

Switch to xenial to fix jdk11 issue for travis

Brief changelog

https://travis-ci.community/t/install-of-openjdk11-is-failing-again/3061/15

Verifying this change

CI pass

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a GITHUB_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GITHUB issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Format the pull request title like [Dubbo-XXX] Fix UnknownException when host config not exist #XXX. Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add integration-test in test module.
• Run mvn clean install -DskipTests=false & mvn clean test-compile failsafe:integration-test to make sure unit-test and integration-test pass.
• If this contribution is large, please follow the Software Donation Guide.

[htynkn]

Travis back to normal. no need for this PR