Fix javadoc with java 11

pom.xml

@@ -184,6 +184,13 @@
           </execution>
         </executions>
       </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-javadoc-plugin</artifactId>
+        <configuration>
+	  <source>8</source>
+        </configuration>
+      </plugin>      
     </plugins>
   </build>