haskell-bifunctors: fix build with GHC 7.6.x

Changed files
+3
pkgs
development
haskell-modules
+3
pkgs/development/haskell-modules/configuration-ghc-7.6.x.nix
···
# Needs additional inputs on pre 7.10.x compilers.
semigroups = addBuildDepends super.semigroups (with self; [bytestring-builder nats tagged unordered-containers transformers]);
+
# Haddock doesn't cope with the new markup.
+
bifunctors = dontHaddock super.bifunctors;
+
}