rpmbuild: use wakame-vdc-ruby.

improve build-required ruby binary setup.

* if exists wakame-vdc-ruby, installing wakame-vdc-ruby package.
* if not, using ruby-build.

```
yum search wakame-vdc-ruby | egrep -q wakame-vdc-ruby && {
  yum install -y wakame-vdc-ruby
} || {
  CURDIR=${CURDIR} make build
}
```