Update Spark version to 2.4.0 (holdenk#271)

* Update Spark version to 2.4.0

* Add Spark version 2.4.0 to `mini-cross-build.pl` file

Author: morazow

build.sbt

@@ -6,7 +6,7 @@ publishMavenStyle := true
 
 version := "0.10.0"
 
-sparkVersion := "2.3.1"
+sparkVersion := "2.4.0"
 
 scalaVersion := {
   if (sparkVersion.value >= "2.0.0") {

mini-cross-build.pl

@@ -2,7 +2,7 @@
 use File::Slurp;
 use strict;
 use warnings;
-my @spark_versions = ("1.4.0", "1.4.1", "1.5.0", "1.5.1", "1.5.2", "1.6.0", "1.6.1", "1.6.2", "1.6.3", "2.0.0", "2.0.1", "2.0.2", "2.1.0", "2.1.1", "2.1.2", "2.1.3", "2.2.0", "2.2.1", "2.2.2", "2.3.0", "2.3.1", "2.3.2");
+my @spark_versions = ("1.4.0", "1.4.1", "1.5.0", "1.5.1", "1.5.2", "1.6.0", "1.6.1", "1.6.2", "1.6.3", "2.0.0", "2.0.1", "2.0.2", "2.1.0", "2.1.1", "2.1.2", "2.1.3", "2.2.0", "2.2.1", "2.2.2", "2.3.0", "2.3.1", "2.3.2", "2.4.0");
 # Backup the build file
 `cp build.sbt build.sbt_back`;
 # Get the original version