Fixes #15628 - Require rdoc for apipie-rails

apipie-rails has rdoc optional, however, its default markup generator is
RDoc. rdoc seems to be found on most of the default OS installations
through inclusion from rubygems.

This fixes (development) setup when rdoc is missing:

```
`require': cannot load such file -- rdoc (LoadError)
```

Technically, the apipie-rails should not fail with default configuraton
when an optional dependency is not installed:
https://github.com/Apipie/apipie-rails/issues/544