我刚刚升级到 ubuntu 14.04，现在 apache2 无法启动，出现错误：
root@dennisHome:~# /etc/init.d/apache2 restart
* Restarting web server apache2
...fail!
* The apache2 configtest failed.
配置测试的输出是：
apache2: Syntax error on line 244 of /etc/apache2/apache2.conf:
Syntax error on line 1 of /etc/apache2/mods-enabled/authz_default.load: Cannot load /usr/lib/apache2/modules/mod_authz_default.so into server:
/usr/lib/apache2/modules/mod_authz_default.so: cannot open shared object file: No such file or directory
Action 'configtest' failed.
The Apache error log may have more information
看起来 mod_authz_default.so 已经不存在了，我该怎么做才能解决这个问题？