Update php version requirement

The package `adambrett/shell-wrapper` uses array literals to define new arrays. This is supported starting in PHP version `5.4.0`.

https://github.com/adambrett/php-shell-wrapper/blob/3237af11ef44f7f1d3df93e55c0ba2741046886c/src/Command/Collections/Arguments.php
http://php.net/manual/de/language.types.array.php#language.types.array.syntax.array-func