Add OpenJDK 8 to Travis CI builds
Add OpenJDK 8 to Travis CI builds

Change .travis.yml to also run the build with OpenJDK 8.

Part of #3470 - Move to using Java 8

psiinon/zaproxy,zaproxy/zaproxy,Ali-Razmjoo/zaproxy,zapbot/zaproxy,gmaran23/zaproxy,zaproxy/zaproxy,psiinon/zaproxy,Ali-Razmjoo/zaproxy,meitar/zaproxy,zaproxy/zaproxy,gmaran23/zaproxy,zapbot/zaproxy,zaproxy/zaproxy,kingthorin/zaproxy,psiinon/zaproxy,gmaran23/zaproxy,thc202/zaproxy,Ali-Razmjoo/zaproxy,meitar/zaproxy,thc202/zaproxy,thc202/zaproxy,thc202/zaproxy,Ali-Razmjoo/zaproxy,gmaran23/zaproxy,gmaran23/zaproxy,meitar/zaproxy,psiinon/zaproxy,kingthorin/zaproxy,psiinon/zaproxy,thc202/zaproxy,kingthorin/zaproxy,psiinon/zaproxy,kingthorin/zaproxy,zaproxy/zaproxy,zapbot/zaproxy,zapbot/zaproxy,meitar/zaproxy,Ali-Razmjoo/zaproxy,kingthorin/zaproxy,Ali-Razmjoo/zaproxy,kingthorin/zaproxy,meitar/zaproxy,thc202/zaproxy,meitar/zaproxy,meitar/zaproxy,zapbot/zaproxy,thc202/zaproxy,Ali-Razmjoo/zaproxy,gmaran23/zaproxy,zapbot/zaproxy,psiinon/zaproxy,zaproxy/zaproxy,zapbot/zaproxy,zaproxy/zaproxy,gmaran23/zaproxy,meitar/zaproxy,kingthorin/zaproxy