Make version a parameter in GHC HEAD derivation

If you want to override the source but the major version changes (ie 8.1
-> 8.3) then you also have to modify the version. Otherwise the build
will fail with difficult to understand errors, making version a
parameter makes it easy to override.

pkgs/development/compilers/ghc/head.nix

@@ -5,6 +5,7 @@
   # If enabled GHC will be build with the GPL-free but slower integer-simple
   # library instead of the faster but GPLed integer-gmp library.
 , enableIntegerSimple ? false, gmp
+, version ? "8.1.20170106"
 }:
 
 let
@@ -12,7 +13,6 @@ let
 
   commonBuildInputs = [ ghc perl autoconf automake happy alex python3 ];
 
-  version = "8.1.20170106";
   rev = "b4f2afe70ddbd0576b4eba3f82ba1ddc52e9b3bd";
 
   commonPreConfigure =  ''