More UNIX-agnostic, less Linux-centric shebang line

Summary:

• Using #!/usr/bin/env <command> as Python could be installed at other places, like /usr/local/bin/python3 on FreeBSD.
• python3 seems the more usual but not universal way to call Python 3.

Test Plan: Tested on FreeBSD 10, CentOS 7 and Debian Jessie.

Reviewers: rama

Projects: Tasacora

Maniphest Tasks: T259

Differential Revision: http://devcentral.nasqueron.org/D7