Temporarily downgrade Diff to 0.3.2

We're having problems building with Diff 0.3.3 on GHC versions older
than 7.10. Until we can resolve those, pin Diff to 0.3.2 so that we can
release Bond 4.1.0.

compiler/bond.cabal

@@ -90,7 +90,7 @@ test-suite gbc-tests
                     derive,
                     HUnit,
                     QuickCheck,
-                    Diff >= 0.2 && < 0.4,
+                    Diff >= 0.2 && <= 0.3.2,
                     pretty,
                     tasty,
                     tasty-golden,