Set the new lucene version for 6.4.0

Author: jimczi

server/src/main/java/org/elasticsearch/Version.java

@@ -171,7 +171,7 @@ public class Version implements Comparable<Version>, ToXContentFragment {
     public static final int V_6_3_0_ID = 6030099;
     public static final Version V_6_3_0 = new Version(V_6_3_0_ID, org.apache.lucene.util.Version.LUCENE_7_3_0);
     public static final int V_6_4_0_ID = 6040099;
-    public static final Version V_6_4_0 = new Version(V_6_4_0_ID, org.apache.lucene.util.Version.LUCENE_7_3_0);
+    public static final Version V_6_4_0 = new Version(V_6_4_0_ID, org.apache.lucene.util.Version.LUCENE_7_4_0);
     public static final int V_7_0_0_alpha1_ID = 7000001;
     public static final Version V_7_0_0_alpha1 =
         new Version(V_7_0_0_alpha1_ID, org.apache.lucene.util.Version.LUCENE_7_4_0);