Travis: downgrade to JDK 8u11 to avoid JDK-8065315

This is a (temporary) workaround to avoid JDK-8065315 bug until Travis
VM image provides again a sane Oracle JDK 8 version.
Merge pull request #6 from gildegoma/fix-travis

Travis CI: downgrade to Oracle JDK 8u11 to avoid JDK-8065315