Upgrade to Maven Javadoc Plugin 3.6.3

Closes gh-413

Author: snicoll

pom.xml

@@ -179,7 +179,7 @@
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-javadoc-plugin</artifactId>
-					<version>3.5.0</version>
+					<version>3.6.3</version>
 				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>