Revert "Download `check-ping.rb` from Github."

Instead, we use Nagios plugins.

This reverts commit d4c6859690c01d09d92058e34824ffa14738cfbf.
This commit refs #51.
Install `nagios-plugins` package.

This commit refs #51.
Use Nagios plugins to check/monitor servers.

This commit refs #41.
Enable `sensu` user to execute the Nagios plugins as root.

This commit refs #51.