Don't use "with licenses;" for single licences

And don't use square brackets on such lines.

pkgs/applications/misc/jekyll/default.nix

@@ -13,7 +13,7 @@ bundlerEnv {
   meta = with lib; {
     description = "Simple, blog aware, static site generator";
     homepage    =  http://jekyllrb.com/;
-    license     = with licenses; mit;
+    license     = licenses.mit;
     maintainers = with maintainers; [ pesterhazy ];
     platforms   = platforms.unix;
   };