Add version for maven-javadoc-plugin, so that Maven does not output warnings about it.
diff --git a/pom.xml b/pom.xml
index fc45919..418c8d6 100644
--- a/pom.xml
+++ b/pom.xml
@@ -499,6 +499,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-javadoc-plugin</artifactId>
+        <version>2.9.1</version>
         <configuration>
           <groups>
             <group>