Upgrade to newer javadoc plugin version and set source to 7 for this

plugin so it will actually build properly with Java 11. There are still
a bunch of new javadoc warnings with Java 11 that I would like to clean
up, but it at least builds now.

Author: davewichers

pom.xml

@@ -419,8 +419,9 @@
             <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-javadoc-plugin</artifactId>
-               <version>3.0.1</version>
+               <version>3.1.0</version>
                <configuration>
+                 <source>7</source>
                  <doclint>none</doclint>
                </configuration>
                <executions>