Fix unreliable Travis builds
Fix unreliable Travis builds

Attempting to enable Java 9 following the advice on
[issue 5520](https://github.com/travis-ci/travis-ci/issues/5520#issuecomment-175914595)
has resulted in the JDK 8 link that JAVA_HOME points to being intermittently deleted.
Alternative advice on
[issue 5897](https://github.com/travis-ci/travis-ci/issues/5897#issuecomment-211062030)
appears to be more reliable.

inferred/FreeBuilder