changed magic line from python->python2

Since python is sometimes (and for me on arch linux) a symlink to the
python3 utils, you should be more explicit with that.

run

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 import os
 import sys

setup.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 # coding=utf-8
 
 from setuptools import setup, find_packages

src/octoprint/__init__.py

@@ -1,4 +1,5 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
+
 import sys
 from octoprint.daemon import Daemon
 from octoprint.server import Server