[jvm-packages] Fix maven warnings (#4664)

* exec plugin was missing a version
* reportPlugins has been deprecated:
  see https://maven.apache.org/plugins/maven-site-plugin/maven-3.html#Classic_configuration_Maven_2__3

jvm-packages/xgboost4j/pom.xml

@@ -54,6 +54,7 @@
           <plugin>
               <artifactId>exec-maven-plugin</artifactId>
               <groupId>org.codehaus.mojo</groupId>
+              <version>1.6.0</version>
               <executions>
                   <execution>
                       <id>native</id>