Upgrade to maven-javadoc-plugin 2.10.4

Release notes:

https://mail-archives.apache.org/mod_mbox/maven-announce/201409.mbox/%3CCA%2BBhkyOw7eio0Mrh0a4mTBtpj%3DKbCv3aZay%2BA6HgYAsPmW_NTg%40mail.gmail.com%3E
https://mail-archives.apache.org/mod_mbox/maven-announce/201410.mbox/%3C542D1009.2030505%40apache.org%3E
https://mail-archives.apache.org/mod_mbox/maven-announce/201503.mbox/%3C20150318202436.4DA8017ECA%40minotaur.apache.org%3E
https://mail-archives.apache.org/mod_mbox/maven-announce/201504.mbox/%3C20150416172048.C262417379%40minotaur.apache.org%3E
https://mail-archives.apache.org/mod_mbox/maven-announce/201606.mbox/%3Cop.yiuy6lohkdkhrr%40desktop-2khsk44%3E

pom.xml

@@ -122,7 +122,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-javadoc-plugin</artifactId>
-        <version>2.9.1</version>
+        <version>2.10.4</version>
         <executions>
           <execution>
             <id>attach-javadocs</id>