Install `cron-apt` package.

This commit refs #15.
Configure `cron-apt`.

- Always output the syslog
- Verbose debug logging
- Do NOT send emails
- Use `aptitude` command, instead of `apt` command
- Only download and install security updates

This commit refs #15.
Merge pull request #15 from kazu634/id/15

cron-aptをインストールする