Workaround for javadoc compile issue

Author: Danny Becker

hadoop-hdfs-project/hadoop-hdfs-client/pom.xml

@@ -190,6 +190,7 @@ https://maven.apache.org/xsd/maven-4.0.0.xsd">
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-javadoc-plugin</artifactId>
         <configuration>
+          <source>8</source>
           <excludePackageNames>org.apache.hadoop.hdfs.protocol.proto</excludePackageNames>
         </configuration>
       </plugin>