nagstamon.py: Let 'env' decide which binary to use

I haphazardly changed this from /usr/bin/env python in a previous
commit. However, this script is not python3 compatible, so I think it's
better to be explicit about the version we require. I'd put in python2.7
but that might be too harsh.

Signed-off-by: Anton Lofgren <alofgren@op5.com>