Update documentation indexer to Lucene 4.4

Change-Id: I129490b25df46b4602456df7464a1fc2c30b676b
diff --git a/lib/asciidoctor/java/DocIndexer.java b/lib/asciidoctor/java/DocIndexer.java
index 1185f5a..5da7858 100644
--- a/lib/asciidoctor/java/DocIndexer.java
+++ b/lib/asciidoctor/java/DocIndexer.java
@@ -40,8 +40,7 @@
 import org.kohsuke.args4j.Option;
 
 public class DocIndexer {
-
-  private static final Version LUCENE_VERSION = Version.LUCENE_43;
+  private static final Version LUCENE_VERSION = Version.LUCENE_44;
   private static final String DOC_FIELD = "doc";
   private static final String URL_FIELD = "url";