Workaround to support JDK 6 in Travis CI
Workaround to support JDK 6 in Travis CI

*Installing JDK* phase is executed before *Installing APT Packages*
phase and when using `jdk: openjdk6` we got an error with Travis
`install-jdk.sh` script.

This workaround start by using an installed JDK (oraclejdk8) and
switch to openjdk6 in `before_install` phase when Installing APT
Packages was already executed.

See: https://travis-ci.community/t/java-jdk-on-travis-ci/1380/2

saeg/ba-dua