Updated to compile in Travis under JDK11 for sonar cloud

.travis.yml

@@ -5,7 +5,7 @@ addons:
         organization: "bentobox-world"
 
 jdk:
-    - openjdk8
+    - openjdk11
     
 script:
   # JaCoCo is used to have code coverage, the agent has to be activated

pom.xml

@@ -278,11 +278,13 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-javadoc-plugin</artifactId>
-                <version>3.0.1</version>
+                <version>3.2.0</version>
                 <configuration>
+                    <source>8</source>
                     <show>public</show>
                     <failOnError>false</failOnError>
                     <additionalJOption>-Xdoclint:none</additionalJOption>
+                    <javadocExecutable>${java.home}/bin/javadoc</javadocExecutable>
                 </configuration>
                 <executions>
                     <execution>