ruby ports: enable parallel builds

[mascguy (Christopher Nielsen)]

It looks like virtually every one of our Ruby ports - with the lone exception of ruby186 - are configured to disable parallel builds.

Quickly searching through all old MacPorts tickets, there don't appear to be any issues related to this. No did 2 minutes of general Googling, return anything suggesting that parallel builds cause issues.

And finally, there is no explanation in the various Ruby portfiles, as to why parallel builds are disabled.

Is there a technical reason for this? Or did this originate as copy-pasta from another port, and ultimately propagate to every Ruby port beyond that?