Reject packages that don't specify an upper bound

[tibbe]

We're seeing an increased number of breakages due to packages not adding upper bounds (see e.g. haskell/HTTP#55). cabal upload ought to reject such packages (or at least give a strong warning). This will not fix the packages that are already uploaded without bounds*, but at least this will help prevent the issue in the future.

*@dcoutts is working on post facto .cabal file editing to help with that problem.

[BardurArantsson]

I think this is more a "hackage server" issue? (Presumably "cabal upload" reports whatever the hackage server reports if there's an error.)