Update PHP version requirement

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