Uncommented accidentaly commented servlet version check

svn changeset:10439/svn branch:6.3

build/build.xml

@@ -134,7 +134,7 @@
       <echo>Java Servlet API is available.</echo>
 
       <available classpathref="compile.classpath" classname="javax.servlet.ServletRequestListener" property="servlet.version.is-2.4"/>
-      <!--<fail if="servlet.version.is-2.4" message="Java Servlet API 2.4 or later detected. Vaadin must be compiled exactly with Servlet API 2.3."/>-->
+      <fail if="servlet.version.is-2.4" message="Java Servlet API 2.4 or later detected. Vaadin must be compiled exactly with Servlet API 2.3."/>
       <echo>Java Servlet API specification 2.3 used.</echo>
     </target>