for PHP 5.5, don't try to profile the builtins

I don't know if this is the best way to fix this problem but XHGui does not work out of the box on PHP 5.5 and I am hearing "XHGui doesn't work on newer vesrsions of PHP".  Actually it's XHProf that's causing the problem, and I've just been using this workaround for my PHP 5.5 projects - maybe there's a reason this isn't added already but in case there isn't, here you go.