java: Bump to Java 8

java: Bump to Java 8

This fixes the following warnings:

    [javac] warning: [options] bootstrap class path not set in conjunction with -source 1.5
    [javac] warning: [options] source value 1.5 is obsolete and will be removed in a future release
    [javac] warning: [options] target value 1.5 is obsolete and will be removed in a future release

Java 8 was released in 2014 so it should be widely available now.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>