Add PHP7.3 and Magento 2.3.3 version requirements

[mirsch]

See #171

[DanielRuf]

@mirsch thanks for your PR =)

This looks good so far.
Can you revert the change of the lockfile? We will update it before we will do the next release.

[mirsch]

@DanielRuf I did not commit the lockfile in first attempt but without an update it breaks travis, because composer validate will fail with The lock file is not up to date with the latest changes in composer.json, it is recommended that you run 'composer update'.
I reverted now, but have to update at least content-hash and platform reqs. to get green light from ci.

[shochdoerfer]

@mirsch looks good to me. Thank you. Will try to get release out in the next few days.