Travis: various tweaks

[jrfnl]

Context

• Update CI

Summary

This PR can be summarized in the following changelog entry:

• Update CI

Relevant technical choices:

Travis: remove nightly

At this time, the current PHP 8.0 version is 8.0.11. The Travis PHP 8.0 image yields PHP 8.0.9, which is good enough for PHP 8.0.

nightly however is PHP 8.0.3 and hasn't been updated since Feb 2021, so running tests against that image is completely useless at this time.

Notes:

• Tested nightly against all relevant dists - xenial, bionic and focal and none have a more current image.
• The PHP Build project, which Travis pulls its images from, does have a PHP 8.1snapshot image available. I've tested to see if that image is available on Travis, but unfortunately, no luck there either.

Travis: minor tweaks

The composer.json file does not fix the PHP version via a config - platform - php setting and there are currently no dependencies which would not allow installation on PHP 8.0, so this condition is redundant.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• N/A if the build passes, we're good