bin/provision-instance: Switch to /usr/bin/env bash

macOS ships with Bash 3.x, where the script fails as follows:

```
$ bin/provision-instance.sh builder-05.stage.ops.jquery.net staging
bin/provision-instance.sh: line 10: VERSION_CODENAME: unbound variable
```

In order to allow use of newer Bash, prefer `env bash`.

Author: Krinkle

bin/provision-instance.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Script to install Puppet on new instances
 
 set -euo pipefail