housekeeping, upgrade packaged JRE to official

- bump release
- rename global build cache dir
- upgrade grade-download-task to get header support
- minor upgrades: snakeyml, commons-codec
- switch to official JRE 8 packages
- download .tgz OS X releases, get rid of .dmg repacking
- update third-party copyright notices
- get rid of some IDE warnings in build.gradle