PHP 8.1 compatibility

[jrfnl]

Hiya!

I just noticed some PHP 8.1 notices related to Mustache for a repo where this package is a dependency of a dependency ;-)

While I'd love to send in some PRs to fix things, but that would require to first get the tests running on PHP 8.1.

Looking at the current setup, I see the following issues:

• CI still on Travis, while Travis is effectively dead for open source.
  👉🏻 Would a PR to switch the existing Travis script over to GH Actions be welcome ?
• PHPUnit limited to "~3.7|~4.0|~5.0".
  Making the tests PHP/PHPUnit cross version compatible to include PHP 8.x with those version constraints is going to be exceedingly hard.
  There is a PHPUnit Polyfills package available which will allow an upgrade to the tests to use PHPUnit 9.x assertions and expectations and get round the PHPUnit 8.x void issue, while still being able to run the tests all the way back to PHPUnit 4.x, but that package requires a minimum PHP version of PHP 5.4, while this package still requires PHP 5.2.

I'd love some guidance on how to proceed.

[jrfnl]

Another thing I noticed: in the Travis script, it gives the impression that HHVM is still supported. Is this really true ?

[jrfnl]

Some quick testing/investigating shows me that the fabpot/yaml test dependency which is pulled in via a submodule is not (fully) compatible with PHP 8.0. As that package is abandoned, that won't be fixed anymore, so a solution for this will also need to be found.