ghc-head: bump version to current git HEAD

pkgs/development/compilers/ghc/head.nix

···
 in
 
 stdenv.mkDerivation rec {
-  version = "7.11.20150705";
+  version = "7.11.20150809";
   name = "ghc-${version}";
-  rev = "3fabb71a559b493efdfb5bb91907f6a0f696a114";
+  rev = "a40ec755d8e020cd4b87975f5a751f1e35c36977";
 
   src = fetchgit {
     url = "git://git.haskell.org/ghc.git";
     inherit rev;
-    sha256 = "1c0fsk2yk08x02n6s722d36igc9prrql4wcnr8m8c62wp4yrf25z";
+    sha256 = "1hh1p9vrd1nrfi56jan4bnlczld2qzx85v7lfb6nara2bhcgqa1l";
   };
 
   postUnpack = ''