cad/ldraw: Fix fetch; distfile was rerolled

I have the idea that ldraw is always released under the same filename.
The port was marked broken due to a checksum mismatch; the file was
fetched but it had changed.

I've expanded DIST_SUBDIR to include the PORTVERSION. I think the scheme
used is just {YEAR}{ITERATION-2-DIGIT} so I picked 201501 as the new
PORTREVISION. This builds under poudriere testport just fine.

Approved by: blanket