ruby_version handling

[Pavel Valena]

Hello,

in Fedora, there's a patch which is supposed to modify upstream behaviour
for paths which are using ruby_version config entry.  Preferably replaced
by the `ruby_version_dir_name` config entry, if it's not empty. This
behaviour was broken (it was checking for `nil`), so I fixed the patch.

This leads to a versioned gem path in `~/.local`.

Please take a look here:

https://src.fedoraproject.org/rpms/ruby/pull-request/151

Regards,
Pavel