Add support for QuickCheck 2.5, 2,6 and 2.7.

Signed-off-by: Peter Simons <simons@cryp.to>
testpack.cabal: strip trailing whitespace
testpack.cabal: document the location of the Git repository
testpack.cabal: point the home page link to Github
testpack.cabal: document the fact that this package is unmaintained
testpack.cabal: bump version number to 2.1.2.1