Bump the Version we use to create/open indexes

Lucene 4.8 adds checksums and Lucene 4.9 adds 'SortedNumeric'. The
former is immediately useful, the latter only with a code change to
use that new type, but we might as well update to the latest here.

src/main/java/com/github/rnewson/couchdb/lucene/util/Constants.java

@@ -22,7 +22,7 @@
 
 public final class Constants {
 
-    public static final Version VERSION = Version.LUCENE_47;
+    public static final Version VERSION = Version.LUCENE_4_9;
 
     public static final Analyzer ANALYZER = new StandardAnalyzer(VERSION);