Upgrade to maven-javadoc-plugin 3.0.1

Notably this fixes issues with newer versions of Java.  Release notes:

https://mail-archives.apache.org/mod_mbox/maven-announce/201712.mbox/%3Cop.zaufhhzukdkhrr%40desktop-2khsk44.mshome.net%3E
https://mail-archives.apache.org/mod_mbox/maven-announce/201805.mbox/%3Cop.zjqrfkbukdkhrr%40desktop-2khsk44%3E

pom.xml

@@ -135,7 +135,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-javadoc-plugin</artifactId>
-        <version>2.10.4</version>
+        <version>3.0.1</version>
         <executions>
           <execution>
             <id>attach-javadocs</id>