Remove upper-bounds on test dependencies

Since the package is on Stackage now, let's remove these for easier
upgrades. The quickcheck 2.7 change would have been a no-op again
anyway…
Lower bounds on hunit/t-f-hunit packages

This allows the package to build and test on Wheezy, yay!
Improve compatibility with QuickCheck >= 2.7

Switch to the new 'counterexample' name (I'm still confused about this
gratuitous rename, but…), but add a fallback mechanism to still
support older QuickCheck versions (tested with 2.4.2, and barring
cabal issues with not defining test-only macros, still works).
Bump version in preparation for release
Rename the CHANGES file

… so that tools parse it indeed as Markdown.
Fix hlint after introducing CPP macros in tests
Apply isNothing lint suggestion
Expand the release notes