Add unreleased version 6.1.5 (#29168)

Author: Christoph Büscher

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

@@ -148,6 +148,8 @@ public class Version implements Comparable<Version> {
     public static final Version V_6_1_3 = new Version(V_6_1_3_ID, org.apache.lucene.util.Version.LUCENE_7_1_0);
     public static final int V_6_1_4_ID = 6010499;
     public static final Version V_6_1_4 = new Version(V_6_1_4_ID, org.apache.lucene.util.Version.LUCENE_7_1_0);
+    public static final int V_6_1_5_ID = 6010599;
+    public static final Version V_6_1_5 = new Version(V_6_1_5_ID, org.apache.lucene.util.Version.LUCENE_7_1_0);
     public static final int V_6_2_0_ID = 6020099;
     public static final Version V_6_2_0 = new Version(V_6_2_0_ID, org.apache.lucene.util.Version.LUCENE_7_2_1);
     public static final int V_6_2_1_ID = 6020199;
@@ -177,6 +179,8 @@ public static Version fromId(int id) {
                 return V_6_2_1;
             case V_6_2_0_ID:
                 return V_6_2_0;
+            case V_6_1_5_ID:
+                return V_6_1_5;
             case V_6_1_4_ID:
                 return V_6_1_4;
             case V_6_1_3_ID: