run bash via env command (#1637)

`#!/usr/bin/env` searches `PATH` for `bash`, and `bash` is not always in `/bin``

Signed-off-by: Felix Buehler <account@buehler.rocks>

.travis/test-example-dockerfiles.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 image="$1"

generate-stackbrew-library.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -Eeuo pipefail
 
 stable_channel='22.2.3'

update.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -eo pipefail
 
 declare -A php_version=(