Travis updates: drop PHP 7.0, add 7.2 and 7.3

[benlk]

https://github.com/INN/largo/blob/f50df16e9e06fb9402802e2f1ce76ce058d26d76/.travis.yml#L60-L68

PHPunit 7 support was added in https://core.trac.wordpress.org/ticket/43218

• [ ] add placeholder for PHP 7.4, and reference https://core.trac.wordpress.org/ticket/46235 for when we should add it to the test matrix
• [ ] remove PHP 7.0 and 7.1
• [ ] write docs for determining how to update these tests, since this will be a yearly task if PHP continues development at its present pace.
  • link to relevant wp.org trac tickets, where known
  • link to https://www.php.net/supported-versions.php
  • link to https://phpunit.de/supported-versions.html

[MirandaEcho]

Travis updates for PHP