cli: Remove shebang

A shebang is present at the beginning of file, it dates from the time
when `yamllint/cli.py` was `bin/yamllint`, i.e. an executable launcher.
Since this is not the case anymore (see `entry_points` section in
`setup.py`), let's remove it.

yamllint/cli.py

@@ -1,4 +1,3 @@
-#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 # Copyright (C) 2016 Adrien Vergé
 #