Fix location of PHP binary in tests/setup shebang

PHP binary can be located in different directories depending on the OS. Using ```#!/usr/bin/env php``` instead of ```#!/usr/bin/php``` avoids problems when, for instance, multiple versions of PHP are installed.

I spent hours trying to figure out why none of oauth modules were not working despite being installed, but it turned out that by calling ```./setup``` I was simply invoking php 5.3 instead of php 5.4 (which, of course, was misconfigured); this was a really difficult issue to spot.

```#!/usr/bin/env php``` is used in Symfony's console, by the way.