Allow PHP 8

[GrahamCampbell]

Q	A
Bug fix?	yes
New feature?	yes
BC breaks?	no
Deprecations?	no
License	MIT

🎉

[GrahamCampbell]

Note that the actions stuff is working correctly, on my fork, however it just won;t show on this PR until the actions config is merged to master. GitHub don't run actions when the target branch (for whatever reason).

[GrahamCampbell]

In particular, I chose actions because of the fact we can write 8.0 for the PHP version. Travis refuse to do this... which is stupid because if we write nightly, we'd only have to update the config later, because we really did want 8.0.

[GrahamCampbell]

NB The actions run on my fork is: https://github.com/GrahamCampbell/promise/actions/runs/160365041. The coverage stuff should work on this repo. My fork just isn't enabled on scrutinizer.

[dbu]

i deleted the travis-ci hook

[GrahamCampbell]

Travis wouldn't run anyway since I deleted its config file from the repo. :)

[dbu]

i assumed it would start to error

[GrahamCampbell]

It just skips builds that have no config. It's useful when a repo has some branches only on travis and some only on actions.