php-xdebug: update to latest version

Fixes http://hydra.nixos.org/job/nixos/trunk-combined/nixpkgs.phpXdebug.x86_64-linux and maybe http://hydra.nixos.org/job/nixos/trunk-combined/nixpkgs.phpXdebug_5_4.i686-linux

https://github.com/NixOS/nixpkgs/pull/661