Change shebang on contrib scripts

contrib/clean_pyc.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Display information
 echo "This script will remove *.pyc files. These files are generated by Python, but they can cause conflicts after an upgrade. It's safe to remove them, because they will be regenerated."

contrib/downgrade.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Parameter check
 if [ -z "$1" ]; then