Rename `ntp_spec.rb` to `ntpdate_spec.rb`.

Rename, because easily associate the chef file (= ntpdate.rb)
to its spec file (= ntpdate_spec.rb).

This commit refs #60.
Rename `package_spec.rb` to `packages_spec.rb`.

Rename, because easily associate the chef file (= `packages.rb`)
to its spec file (= `packages_spec.rb`).

This commit refs #60.
`packages_spec.rb` is responsible for `Ruby` package installation.

According to the chef file (= `packages.rb`) responsibilities,
`packages_spec.rb` is responsible for `Ruby` package installation.

This commit refs #60.