move to Java 8

basically just to be consistent with Tortoise and Galapagos repos
and generally move to using the latest tools while Tortoise is
still young and upgrades like this are still low-impact
move extension builds to Java 8
update benchmark dumps for Java 8
Merge remote-tracking branch 'origin/headless' into jdk8
Merge pull request #572 from NetLogo/jdk8

move to Java 8