Fix shebangs from /bin/bash to /bin/sh

These simple scripts do not use any bash features, so do not
require needless extra dependency

Author: AMDmi3

script/bootstrap

@@ -1,3 +1,3 @@
-#! /bin/bash
+#!/bin/sh
 
 bundle install

script/cibuild

@@ -1,4 +1,4 @@
-#! /bin/bash
+#!/bin/sh
 set -e
 
 bundle exec rspec