Temporarily revert default Spark version to reenable cross compile

Author: srowen

build.sbt

@@ -10,7 +10,7 @@ crossScalaVersions := Seq("2.11.12", "2.12.10")
 
 scalacOptions := Seq("-unchecked", "-deprecation")
 
-val sparkVersion = sys.props.get("spark.testVersion").getOrElse("3.1.2")
+val sparkVersion = sys.props.get("spark.testVersion").getOrElse("2.4.8")
 
 // To avoid packaging it, it's Provided below
 autoScalaLibrary := false