Better implementation for .phpbrewrc. No more cd hooks.

As promised in #224, here is another bash script with no more cd hooks that could solve #223 in a more elegant way.

Now I'm using a trap DEBUG which is completely non unobtrusive and won't break rvm or any other script that overwrites builtin bash functions. Yuck!

TODO:
- support for zsh

PS: #224 was merged by accident, sorry.