fix warning

commafeed-server/src/main/java/com/commafeed/frontend/servlet/RobotsTxtDisallowAllServlet.java

@@ -10,6 +10,8 @@ import javax.servlet.http.HttpServletResponse;
 @Singleton
 public class RobotsTxtDisallowAllServlet extends HttpServlet {
 
+	private static final long serialVersionUID = 1L;
+
 	@Override
 	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
 		resp.setContentType("text/plain");