Fix runtime failure of redpen server

RedPen server failed with the following log. This patch fixes the runtime error.

    redpen/redpen-server/target$ java -jar redpen-server.war
    Exception in thread "main" java.lang.NoClassDefFoundError: javax/servlet/ServletContext
        at java.lang.Class.getDeclaredMethods0(Native Method)
        at java.lang.Class.privateGetDeclaredMethods(Class.java:2688)
        at java.lang.Class.getMethod0(Class.java:2937)
        at java.lang.Class.getMethod(Class.java:1771)
        at sun.launcher.LauncherHelper.validateMainClass(LauncherHelper.java:544)
        at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:526)
    Caused by: java.lang.ClassNotFoundException: javax.servlet.ServletContext
         at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        ... 6 more