Fix jetty plugin version at 6.1.6.

samples/tutorial/pom.xml

@@ -39,6 +39,7 @@
             <plugin>
                 <groupId>org.mortbay.jetty</groupId>
                 <artifactId>maven-jetty-plugin</artifactId>
+                <version>6.1.6</version>
                 <configuration>
                     <contextPath>/tutorial</contextPath>
                 </configuration>