Switch to GH Actions

[Seldaek]

• Removes Travis CI which is now kinda defunct..
• Added GH actions for CI, code coverage, php cs check
• Added PHPStan too (basic level 2 for now)

PHP 8 is still not built because it'd require major test suite refactorings to get it to work (as we can't run phpunit 4.x on PHP 8).

[guilliamxavier]

Seems to include #655, maybe better merge it first?

[Seldaek]

@guilliamxavier thanks, did not see that one.

[Seldaek]

OK I think for me this is ready to merge now. As a next step we could increase the phpstan level but I wanted to keep it low for now so this is easy to cherry-pick into the 5.x.x branch, so we get at least some basic static analysis going there, and can do the level 8 stuff only on master branch.

[erayd]

Looks fairly sane, although worth noting that I'm not overly familiar with deploying GH actions, so if there are problems in this, it's entirely possible I may have missed them.

[erayd]

This was pinned due to an issue with newer minor versions of php-cs-fixer rewriting code in a way that actually caused logic changes, without warning. I would prefer to continue with pinning specific versions if possible, as this happened on multiple occasions with different versions of php-cs-fixer.

Is there are strong reason that requires loosening this constraint?

[Seldaek]

The main reason was allowing it to install fine on PHP 8 in development, because 2.15 requires php 5/7. But I'm happy to pin it to 2.19 instead to avoid surprises in the future.

[Seldaek]

Thanks for the review, merging this then, which should hopefully help getting future PRs checked more easily.

[sanmai]

it'd require major test suite refactorings to get it to work

Done in #693