Add package PHP version requirement

[peter279k]

array_undot/composer.json

Line 5 in 44aed52

"require-dev": {

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 😃.

[DivineOmega]

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