PHP 7.1 dependency

[DrillSergeant]

Hey Sandstrom guys,

I just wanted to try out v1.x of this package. As there are return type declaration of void is used (at least in Domain\Model\Content::setSource and mabye more) I cannot run it because we use Neos 3.3 with PHP 7.0.33 on our servers.

As the 1.x branch is for Neos 3.x and Neos 3.x only requires php: ~7.0 I suggest to remove that void return type declaration.

If that is not possible I suggest to at least add the PHP-version constraint to this package.

Best Regards,
Peter