Bump PORTREVISION again due to ilmbase and openexr update

Reason is that graphics/darktable itself had a commit by dumbbell@
(r544820) that also bumped PORTREVISION=2 but that happened before
I'd committed the OpenEXR updates, and the svn update just merged
that change.

So, unfortunately, we need another PORTREVISION update.

See r544849, r544853.