leave revision 0 line in Portfile

[Ken Cunningham]

I would like to propose we stop asking people to remove the "revision 0" line in Portfiles. 

Rationale:

Functionally the same either way.
Insignificant size.
it's properly located already. This takes thoght, why waste it for next time?
It's petty and bikesheddy to ask everyone to remove it. Sends wrong message.
At some point we are going to want an automated tool to propose version bumps and do dependency revbumps; we'll appreciate every port having that line in then. 

So how about we embrace the "0" and move on to stuff that matters more!!!

K