apache2.4.23 -> 2.4.25

In the case of Ubuntu 16.04 LTS, the version of apache 2 is old. It is apache 2.4.23, but since there is also a security hole (CVE-2016-4979), change to the latest version of apache2.
The procedure is as follows.

$ apache2 -v
Server version: Apache/2.4.23 (Ubuntu)
Server built: 2016-07-11T00:00:00

$ sudo add-apt-repository ppa:ondrej/apache2
$ sudo apt-get update
$ sudo apt-get upgrade

$ apache2 -v
Server version: Apache/2.4.25 (Ubuntu)
Server built: 2016-12-21T09:13:44