specify unidocGenjavadocVersion of 0.8

Fixes an issue with being too strict generating javadoc causing errors.

Author: Holden Karau <holden@pigscanfly.ca>

Closes #2893 from holdenk/SPARK-3359-sbtunidoc-java8 and squashes the following commits:

9379a70 [Holden Karau] specify unidocGenjavadocVersion of 0.8