Add PHP 7.3 to .travis.yml; Remove 7.0

[benlk]

• Add PHP 7.3 to the Travis CI build Matrix for the following WordPress versions:
  • 5.0
  • 4.9.9 if it's released, if it contains PHP 7.3 support
• Remove PHP 7.0
• get the Gutenberg/Classic Editor tests to work, from Are there tests that need to be run with and without Gutenberg? #1514

[benlk]

This is blocked by Travis not yet supporting PHP 7.3 yet; their latest build environment doesn't have PHP 7.3 yet. It only has 5.6, 7.1, and 7.2.

[benlk]

If we want PHP 7.3, we will need to switch to the Xenial build environment: https://docs.travis-ci.com/user/reference/bionic/#php-support

This will require dropping PHP 5.6 support, which we should be okay with doing because PHP 5.6 went end-of-life at the end of 2018: https://www.php.net/eol.php

[benlk]

Add PHP 7.3 to travis tests

[benlk]

Closing in favor of #1801, which is more up-to-date.