quote homepages for better clickability

Done while I was traversing packages which I maintain to save extra clicks on
urxvt (it captures semicolon as a part of URL).

pkgs/servers/mail/dspam/default.nix

@@ -99,7 +99,7 @@ in stdenv.mkDerivation rec {
   '';
 
   meta = with lib; {
-    homepage = http://dspam.nuclearelephant.com/;
+    homepage = "http://nuclearelephant.com/";
     description = "Community Driven Antispam Filter";
     license = licenses.agpl3;
     platforms = platforms.linux;