databases/ruby-bdb uses WITH_BDB6_PERMITTED

[Fernando Apesteguía]

Hi there ruby maintainers!

I'm in the process of updating the Uses=bdb section in the Porter's
Handbook.
While having a look at Mk/Uses/bdb.mk and inspecting the ports tree, I saw
that databases/ruby-bdb is the only port using the WITH_BDB6_PERMITTED hack.
Why is that? Would it be possible to update the port to Uses=bdb:5 or
Uses=bdb:18?

Cheers!