hspec-expectations.cabal: Disable redundant-constraint warnings

GHC 8.0.1 added a new `redundant-constraints` warning that is part of
`-Wall` by default. However, this warning is triggered on code that
needs the constraints (eg AMP with ghc < 7.10) for earlier compilers.

Author: erikd

hspec-expectations.cabal

@@ -25,6 +25,8 @@ library
   hs-source-dirs:
       src
   ghc-options: -Wall
+  if impl(ghc >= 8.0)
+    ghc-options: -fno-warn-redundant-constraints
   build-depends:
       base == 4.*
     , HUnit