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

Author: holdenk

project/SparkBuild.scala

@@ -22,6 +22,7 @@ import sbt._
 import sbt.Classpaths.publishTask
 import sbt.Keys._
 import sbtunidoc.Plugin.genjavadocSettings
+import sbtunidoc.Plugin.UnidocKeys.unidocGenjavadocVersion
 import org.scalastyle.sbt.ScalastylePlugin.{Settings => ScalaStyleSettings}
 import com.typesafe.sbt.pom.{PomBuild, SbtPomKeys}
 import net.virtualvoid.sbt.graph.Plugin.graphSettings
@@ -116,6 +117,7 @@ object SparkBuild extends PomBuild {
     retrieveManaged := true,
     retrievePattern := "[type]s/[artifact](-[revision])(-[classifier]).[ext]",
     publishMavenStyle := true,
+    unidocGenjavadocVersion := "0.8",
 
     resolvers += Resolver.mavenLocal,
     otherResolvers <<= SbtPomKeys.mvnLocalRepository(dotM2 => Seq(Resolver.file("dotM2", dotM2))),