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

@tbsalling This pull request is related to your [question on StackOverflow](http://stackoverflow.com/questions/27047496/javac-1-8-0-25-has-known-bug-how-to-use-different-version).

This is a (temporary) workaround to avoid JDK-8065315 bug until Travis VM image provides again a sane Oracle JDK 8 version.

Note that this trick could be improved by checking which `java -version` is currently installed on the Travis VM, and only downgrade when needed.