Remove PHP 7.0 as the Composer platform requirement

[danielbachhuber]

In #418, we had to add the following as a platform requirement in composer.json:

"config": {
    "platform": {
        "php": "7.0"
    }
},

We did this because WordPress.org's linter is stuck on PHP 7.0. It doesn't seem to be a huge deal though. I think the only downgraded dependencies are PHPUnit (to v6.5.1 from v7) and symfony/event-dispatcher (to v3.3.6 from v4)

Once the linter is upgraded/fixed in https://meta.trac.wordpress.org/ticket/3791, we should remove/update the platform requirement.

Previously #412

[extremeshok]

thank you