`subprocess.Popen` fails when Python is in "App Paths"

Win32. Python is not added to PATHs by default. Instead it is added to `App Paths` registry key ( http://msdn.microsoft.com/en-us/library/windows/desktop/ee872121(v=vs.85).aspx ). In both cases Python binary is found when i enter `python` command. But `subprocess.Popen` seems to fail when BOTH (1) runs a string `python tools/atom_source_root.py` AND (2) python is not in PATHs, only in App Paths. I suggest using list in <!( and hope this does not break linux/macos builds.
Merge pull request #867 from IgorKlopov/patch-2

`subprocess.Popen` fails when Python is in "App Paths"