allow composer-installers version 2

[saschanos]

Fixes #22

[domtra]

@saschanos Thanks a lot for this pull request.
Although we are using composer 2 already with the current version, making this compatible with composer/installers 2.0 is great.

As you can see, the travis build is failing, unfortunately. This should not be the case, as you specifically added "~1.0|~2.0" as a version requirement. (Btw. I think this is old syntax and ~1.0 || ~2.0 should probably be preferred).
The reason it is failing is probably because the committed composer.lock file requires composer/installers 2.0 and this is not compatible with older PHP versions.

Do you have an idea how to fix this? In the CI, we could delete the composer.lock before installing but i do not know if this will cause stability issues. Or we could commit 2 lock files that we switch depending on the PHP version in the CI. Unfortunately, I did not find an elegant, or actually any, solution anywhere. So if you have an idea, I would appreciate it. Otherwise I will just pick one if the two described solutions.

[saschanos]

@domtra, thank you for your feedback :).

What's the reason for maintaining this package for php versions <= 7.1?

These are already out of support and should not be used by any php developer anymore (see: https://www.php.net/supported-versions.php). A lot of other libraries already dropped support for these versions. Would this be an option? We then should also add php 8.0 / php 8.1 to travis CI :).

[saschanos]

@domtra, would be great to get a short reply here :)

[saschanos]

@domtra, would be great to get some feedback here :)

[domtra]

@saschanos , i took your pr, adjusted it a bit and added some more code for ci, etc. thanks for your contribution again.

[domtra]

see #26