upgrade javadoc plugin
diff --git a/pom.xml b/pom.xml
index 8524115..3f8e4b8 100644
--- a/pom.xml
+++ b/pom.xml
@@ -98,7 +98,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-javadoc-plugin</artifactId>
-          <version>2.6.1</version>
+          <version>2.8.1</version>
           <configuration>
                     <source>1.5</source>
                     <target>1.5</target>