Fix to calling the php binary

Summary: by default it was looking for php to be in /usr/bin/php on some systems
this is incorrect. It should be using /usr/bin/env php for calling php with the
correct path.

Test Plan: patch and try to run.

Reviewers: epriestley

Reviewed By: epriestley

CC: svemir, aran, epriestley

Differential Revision: 1136