manage.py: Restore shebang line

This is an executable file but cannot be executed without the interpreter.

The shebang line was lost in commit 8cbc6b8325c08bb3a72932c7e67c6476f7d29edb

lemur/manage.py

@@ -1,3 +1,4 @@
+#!/usr/bin/env python
 from __future__ import unicode_literals  # at top of module
 
 import os