Update xstream docstring version (#607)

Author: peter-lawrey

src/main/java/net/openhft/chronicle/map/JsonSerializer.java

@@ -30,7 +30,7 @@ private JsonSerializer() {
                     "<dependency>\n" +
                     " <groupId>com.thoughtworks.xstream</groupId>\n" +
                     " <artifactId>xstream</artifactId>\n" +
-                    " <version>1.4.20</version>\n" +
+                    " <version>1.4.21</version>\n" +
                     "</dependency>\n" +
                     "<dependency>\n" +
                     " <groupId>org.codehaus.jettison</groupId>\n" +