Upgrade to Java 8

Install the latest Java 8 instead of Java 7.

Since the first release of Java 8 has no update version, I have removed the `update_version` parameter. This will break existing code, that sets the parameter manually. Once, Oracle will release the first update to Java 8, the parameter could be introduced again.

I would recommend to release this as version 2.0.0, due to the broken backwards compatibility and the new Java version.

Someone must upload Java 8 [JDK](http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html) and [JRE](http://www.oracle.com/technetwork/java/javase/downloads/jre8-downloads-2133155.html) to https://s3.amazonaws.com/boxen-downloads/java to make this really work.