Always use #!/usr/bin/env python

Hardcoding the path to the python binary breaks virtualenv. virtualenv
creates a special python binary in the path, and specifically using
/usr/bin/python breaks that.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>