Turn more licenses into lib.licenses style

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

Relevant: issue #2999, issue #739

pkgs/applications/audio/vimpc/default.nix

@@ -28,7 +28,7 @@ stdenv.mkDerivation rec {
   meta = {
     description = "A vi/vim inspired client for the Music Player Daemon (mpd).";
     homepage = https://github.com/boysetsfrog/vimpc;
-    license = "GPL3";
+    license = stdenv.lib.licenses.gpl3;
     platforms = stdenv.lib.platforms.linux;
   };
 }