Use Lucene version string in main rest action

This commit removes the deprecated constant for the main
version and uses the real lucene version we are running instead.
Behind the scenes the same value was used and is now obsolet.

src/main/java/org/elasticsearch/rest/action/main/RestMainAction.java

@@ -82,11 +82,7 @@ public class RestMainAction extends BaseRestHandler {
                 .field("build_hash", Build.CURRENT.hash())
                 .field("build_timestamp", Build.CURRENT.timestamp())
                 .field("build_snapshot", version.snapshot)
-                        // We use the lucene version from lucene constants since
-                        // this includes bugfix release version as well and is already in
-                        // the right format. We can also be sure that the format is maitained
-                        // since this is also recorded in lucene segments and has BW compat
-                .field("lucene_version", Constants.LUCENE_MAIN_VERSION)
+                .field("lucene_version", version.luceneVersion.toString())
                 .endObject();
         builder.field("tagline", "You Know, for Search");
         builder.endObject();