Update cabal version spec for GHC 6.12
Update cabal version spec for GHC 6.12

Fixes #201

GHC 6.12 introduced an incompatibility; Setup.hs can now work with it
or with 6.10 and earlier, and not with both.  Updating the .cabal file
to note the Cabal version required.

hdbc/hdbc-postgresql,cabrera/hdbc-postgresql,beni55/hdbc-postgresql