SCRIPTS: use /usr/bin/env bash instead of /bin/bash for scripts

Given that some OSes have bash in /usr/local/bin and in order not to
give too easy an excuse to Olivier for not backporting fixes, let's
make a few scripts rely on /usr/bin/env bash instead of /bin/bash :-)

scripts/create-release

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # creates a new haproxy release at the current commit
 # Copyright (c) 2006-2016 Willy Tarreau <w@1wt.eu>
 #