apache-httpd: Make 2.4 the default

The NixOS 14.11 release is a good time to finally make 2.4 the
default.

nixos/modules/services/web-servers/apache-httpd/default.nix

@@ -109,6 +109,8 @@ let
       "mpm_${mainCfg.multiProcessingModule}"
       "authz_core"
       "unixd"
+      "cache" "cache_disk"
+      "slotmem_shm"
     ]
     ++ (if mainCfg.multiProcessingModule == "prefork" then [ "cgi" ] else [ "cgid" ])
     ++ optional enableSSL "ssl"
@@ -420,8 +422,7 @@ in
 
       package = mkOption {
         type = types.package;
-        default = pkgs.apacheHttpd.override { mpm = mainCfg.multiProcessingModule; };
-        example = literalExample "pkgs.apacheHttpd_2_4";
+        default = pkgs.apacheHttpd;
         description = ''
           Overridable attribute of the Apache HTTP Server package to use.
         '';
@@ -593,11 +594,11 @@ in
   ###### implementation
 
   config = mkIf config.services.httpd.enable {
-  
+
     assertions = [ { assertion = mainCfg.enableSSL == true
                                -> mainCfg.sslServerCert != null
                                     && mainCfg.sslServerKey != null;
-                     message = "SSL is enabled for HTTPD, but sslServerCert and/or sslServerKey haven't been specified."; }
+                     message = "SSL is enabled for httpd, but sslServerCert and/or sslServerKey haven't been specified."; }
                  ];
 
     users.extraUsers = optionalAttrs (mainCfg.user == "wwwrun") (singleton