Specify source level in javadoc phase, fixes #31
Specify source level in javadoc phase, fixes #31

Also updates to a newer version of the javadoc plugin. Note that
building the javadoc under JDK 11 is highly recommended, because it
adds a lovely search capability!

DerekCook/CoreMidi4J,DerekCook/CoreMidi4J,DerekCook/CoreMidi4J