Remove redundant flags and deps from cabal file

cabal-install no longer builds with GHC 6.8 or base < 4, so we don't
need to test for those configurations.
Reformat and sort dependencies
Since directory 1.2, no longer need old-time
Merge pull request #1232 from benmachine/no-old-time

Since directory 1.2, no longer need old-time
More help string improvements.