Allow use of older QuickCheck for HP compatibility

Trying to install `tasty-quickcheck` on an older version of the Haskell Platform is pretty difficult right now since it comes bundled with `QuickCheck-2.6`, but `tasty-quickcheck` requires >= 2.7. To get around this, I lowered the `QuickCheck` lower version bounds and used CPP pragmas where needed so that older `QuickCheck`s can be used.