Specify a version for Maven Plugin Plugin. This should solve the site build failure in Jenkins.

git-svn-id: https://svn.apache.org/repos/asf/creadur/whisker/trunk@1633932 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/apache-whisker-maven-plugin/pom.xml b/apache-whisker-maven-plugin/pom.xml
index 482d90f..c1749f3 100644
--- a/apache-whisker-maven-plugin/pom.xml
+++ b/apache-whisker-maven-plugin/pom.xml
@@ -174,6 +174,7 @@
             <plugin>
               <groupId>org.apache.maven.plugins</groupId>
               <artifactId>maven-plugin-plugin</artifactId>
+              <version>${mavenPluginPluginVersion}</version>
             </plugin>
             <plugin>
               <groupId>org.apache.maven.plugins</groupId>