Add <source> config to maven-javadoc-plugin

This fixes errors when building for JDK 11 and 12 https://bugs.openjdk.java.net/browse/JDK-8212233

Author: kevinvincent

pom.xml

@@ -127,6 +127,7 @@
         <version>3.1.1</version>
         <configuration>
           <doclint>none</doclint>
+          <source>8</source>
         </configuration>
         <executions>
           <execution>