Use uniform shebang in all demos

Demo scripts used different style of shebangs. The shebang
"#!/usr/bin/env python" is preferred because it works nicely in virtual
envs.

https://github.com/python-ldap/python-ldap/pull/97
Signed-off-by: Christian Heimes <cheimes@redhat.com>

Demo/pyasn1/sessiontrack.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 """
 demo_track_ldap_session.py

Demo/pyasn1/syncrepl.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 """
 This script implements a syncrepl consumer which syncs data from an OpenLDAP

Demo/simplebrowse.py

@@ -1,4 +1,4 @@
-#! python
+#!/usr/bin/env python
 
 #
 # simple LDAP server browsing example