swift234

Invoke "/usr/bin/env bash" instead of "/bin/bash".

Prior to this commit:

```
$ git grep '#!/usr/bin/env bash' | wc -l
8
$ git grep '#!/bin/bash' | wc -l
1
```

After to this commit:

```
$ git grep '#!/usr/bin/env bash' | wc -l
9
$ git grep '#!/bin/bash' | wc -l
0
```