git-annex: switch to the version built with GHC 7.10.1

Also, this change renames the "gitAnnex" attribute to "git-annex". This
seems more in-line with recent additions to the git package family,
which all used dashed names instead of camel-caps.

pkgs/applications/version-management/git-and-tools/default.nix

@@ -46,7 +46,7 @@ rec {
     sendEmailSupport = !stdenv.isDarwin;
   };
 
-  gitAnnex = pkgs.haskell-ng.packages.ghc784.git-annex;
+  inherit (pkgs.haskellPackages) git-annex;
 
   qgit = import ./qgit {
     inherit fetchurl stdenv;