Add package PHP version requirement

[peter279k]

https://github.com/DivineOmega/array_undot/blob/44aed525e775718e3821d670b08046fd84914d10/composer.json#L5

It seems that the require block is not existed on composer.json file.

Is it necessary to add PHP version requirement for this package :)?

And I think it can let developers know what PHP versions can be proper to use for this package :smiley:.

[DivineOmega]

Probably a good idea. Makes sense to move this to PHP 7.1 or above.