Compatibility with Sylius 1.12

[Prometee]

Hello,

While testing this plugin with sylius/sylius-standard:1.12.x-dev, I was unable to install it because of the strict restriction of this plugin composer.json file.

Solution :

Remove the strict requirements for Sylius and replace it by : ^1.9

Also it's maybe useless to specify the php version inside a plugin unless there is a specific reason because Sylius is already requiring a specific PHP version.

Finally, the requirement to symfony/event-dispatcher is a duplicate because Sylius already need it : composer why symfony/event-dispatcher

[mamazu]

Thanks a lot for contributing the update to the project.