Tweaked proc_open() call for callgraph to work on Windows

**Not** tested on Linux, just got it working on Windows with some trial and error.

1. Replaced hardcoded `/tmp` with `sys_get_temp_dir()`, there is some [usual lively discussion in PHP docs](http://php.net/manual/en/function.sys-get-temp-dir.php), might or might not be reliable func to use.

2. Passing empty `array()` for environment argument is different from default `null` and makes it fail to recognize `dot` command on Windows. I am not sure if passing it served some purpose or just unnecessary leftover.