Disable some stuff that won't work with 7.6 anyway.

Closes https://github.com/peti/nixpkgs/pull/19.

pkgs/development/haskell-modules/configuration-ghc-7.6.x.nix

···
   # blaze-builder requires an additional build input on older compilers.
   blaze-builder = addBuildDepend super.blaze-builder super.bytestring-builder;
 
+  # available convertible package won't build with the available
+  # bytestring and ghc-mod won't build without convertible
+  convertible = markBroken super.convertible;
+  ghc-mod = markBroken super.ghc-mod;
+
 }