genjavadoc issue with StreamShapeLowPriority

[lrytz]

Recent travis build: https://travis-ci.org/scala/scala-java8-compat/builds/582046003

The 2.12.8 and 2.11.12 versions show:

[error] /home/travis/build/scala/scala-java8-compat/target/java/scala/compat/java8/StreamShapeLowPriority.java:3:1:  error: illegal combination of modifiers: public and protected
[error]   protected abstract class BaseStreamShape<T extends java.lang.Object, S extends java.util.stream.BaseStream<?, S>, St extends scala.compat.java8.collectionImpl.Stepper<?>> implements scala.compat.java8.StreamShape<T, S> {
[error]                      ^

The generated file: https://gist.github.com/lrytz/899f08e00d48ca14a4943bcfe834e38e

[SethTisue]

This is still broken. In #267, I'm making it so CI catches the problem (by calling publishLocal), so we have a guard against removing the check that restricts doc generation to Java 8