Bumped the (strict) upper bound on the QuickCheck dependency from 2.6 to 2.7

This patch bumps the (exclusive) upper version bound on QuickCheck from 2.6 to 2.7.  Please accept this patch because at this time test-framework-quickcheck2 is not reliably cabal-installable as it requires that an older version of QuickCheck be installed than that in that latest Haskell Platform.