Update maven-javadoc-plugin to fix javadoc error with openjdk 11.0.2 (#59)

Author: koichirok

pom.xml

@@ -134,7 +134,7 @@
                     <plugin>
                         <groupId>org.apache.maven.plugins</groupId>
                         <artifactId>maven-javadoc-plugin</artifactId>
-                        <version>2.10.3</version>
+                        <version>3.1.1</version>
                         <executions>
                             <execution>
                                 <id>attach-javadocs</id>