Specify minimum build JDK of 1.8
Specify minimum build JDK of 1.8

This gives a more useful message explaining why the build fails as
we are now relying on Java 8's `-h` flag for `javac` to build the JNI
headers (for compatibility with JDK 10 and later).

DerekCook/CoreMidi4J,DerekCook/CoreMidi4J,DerekCook/CoreMidi4J