R: don't restrict hydraPlatforms

The restriction of hydraPlatforms was added in [1: ef05fad51a], but
doesn't seem to be a real reason behind it. R is Free Software and it
take takes a few minutes to build so I believe it can be distributed by
Hydra.

1: 2014-05-04 ef05fad51a
   R: don't restrict meta.platforms to Linux; other architectures should build fine

pkgs/applications/science/math/R/default.nix

@@ -118,7 +118,6 @@ stdenv.mkDerivation rec {
     '';
 
     platforms = platforms.all;
-    hydraPlatforms = platforms.linux;
 
     maintainers = with maintainers; [ peti ] ++ teams.sage.members;
   };

pkgs/development/r-modules/default.nix

@@ -22,7 +22,7 @@ let
         doCheck ? true,
         requireX ? false,
         broken ? false,
-        hydraPlatforms ? R.meta.hydraPlatforms
+        hydraPlatforms ? R.meta.platforms
       }: buildRPackage {
     name = "${name}-${version}";
     src = fetchurl {