Improved portability by using /usr/bin/env pragma

[Swivelgames]

I'm not sure if this is desired, but I noticed a lot of inconsistencies throughout the code base with regard to pragma usage.

This updates all uses of #!/bin/bash to use the more portable #!/usr/bin/env bash.

[Swivelgames]

Tested using fidian/multishell-small

bash-3

- Runs error-free: **YES** - Test Failures: _**NO**_ ``` # run-script-againt-shells bash-3 ./run-tests bash-3.2.57: pass bash-3.1.23: FAIL ... tests/function-args ... FAIL (status code 1) # EXISTING TEST ... Pass: 27 Fail: 2 ```

bash-4

- Runs error-free: **YES** - Test Failures: _**NO**_ ``` bash-4.0.44: pass bash-4.1.17: pass bash-4.2.53: pass bash-4.3.48: pass bash-4.4.23: pass ```

bash-5

- Runs error-free: **YES** - Test Failures: _**NO**_ ``` bash-5.0.18: pass bash-5.1.8: pass ```

[fidian]

Thanks, this is great!