:rocket: Build/Travis: test builds against PHP 7.3
:rocket: Build/Travis: test builds against PHP 7.3

Once PHP 7.3-beta came out, the `nightly` build on Travis became PHP 7.4-dev and builds haven't been tested against PHP 7.3 for months now.

As of this week, Travis has (finally) made a PHP 7.3 alias available now RC3 is out, so I've added PHP 7.3 to the matrix.

Yoast/yoastcs