Add 6.8 version constants

This commit adds the 6.8 version constants and bumps the current version
on the 6.8 branch to 6.8.0.

Author: jasontedor

buildSrc/version.properties

@@ -1,4 +1,4 @@
-elasticsearch     = 6.7.2
+elasticsearch     = 6.8.0
 lucene            = 7.7.0
 
 # optional dependencies

docs/Versions.asciidoc

@@ -1,8 +1,8 @@
-:version:               6.7.1
+:version:               6.8.0
 :major-version:         6.x
 :lucene_version:        7.7.0
 :lucene_version_path:   7_7_0
-:branch:                6.7
+:branch:                6.8
 :jdk:                   1.8.0_131
 :jdk_major:             8
 :build_flavor:          default

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

@@ -214,8 +214,10 @@ public class Version implements Comparable<Version>, ToXContentFragment {
     public static final Version V_6_7_1 = new Version(V_6_7_1_ID, org.apache.lucene.util.Version.LUCENE_7_7_0);
     public static final int V_6_7_2_ID = 6070299;
     public static final Version V_6_7_2 = new Version(V_6_7_2_ID, org.apache.lucene.util.Version.LUCENE_7_7_0);
+    public static final int V_6_8_0_ID = 6080099;
+    public static final Version V_6_8_0 = new Version(V_6_8_0_ID, org.apache.lucene.util.Version.LUCENE_7_7_0);
 
-    public static final Version CURRENT = V_6_7_2;
+    public static final Version CURRENT = V_6_8_0;
 
     static {
         assert CURRENT.luceneVersion.equals(org.apache.lucene.util.Version.LATEST) : "Version must be upgraded to ["
@@ -228,6 +230,8 @@ public static Version readVersion(StreamInput in) throws IOException {
 
     public static Version fromId(int id) {
         switch (id) {
+            case V_6_8_0_ID:
+                return V_6_8_0;
             case V_6_7_2_ID:
                 return V_6_7_2;
             case V_6_7_1_ID: