Fix javadoc plugin on later Java versions

See https://bugs.openjdk.java.net/browse/JDK-8212233

Author: barbeau

pom.xml

@@ -88,6 +88,9 @@
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-javadoc-plugin</artifactId>
                 <version>2.10.4</version>
+                <configuration>
+                    <source>8</source>
+                </configuration>
                 <executions>
                     <execution>
                         <id>attach-javadocs</id>