Turn more licenses into lib.licenses style

Should eval cleanly, as far as -A tarball tells me.

Relevant: issue #2999, issue #739

pkgs/development/tools/analysis/spin/default.nix

@@ -20,7 +20,7 @@ stdenv.mkDerivation rec {
   meta = with stdenv.lib; {
     description = "Formal verification tool for distributed software systems";
     homepage = http://spinroot.com/;
-    license = "free";
+    license = stdenv.lib.licenses.free;
     maintainers = with maintainers; [ mornfall pSub ];
   };
 }