As it was, the plugin when used by external standards, would block them from being tested on PHP 8.

As all the dependencies in the `require` section allow for PHP 8, there is no reason not to allow it.

The only dependencies currently in use which do not allow for PHP 8 yet are in the `require-dev` section. See the below output.

This includes updating the Travis script to only do a full (`dev`) install when not on `nightly` and as PHPCompatibility is in the `require-dev` section, we need to require explicitly for `nightly` to make sure the integration test can run.

```bash
# composer prohibits php:8

composer/composer          1.9.3   requires  php (^5.3.2 || ^7.0)
composer/semver            1.5.1   requires  php (^5.3.2 || ^7.0)
seld/jsonlint              1.7.2   requires  php (^5.3 || ^7.0)
symfony/console            v4.4.4  requires  php (^7.1.3)
symfony/filesystem         v4.4.4  requires  php (^7.1.3)
symfony/finder             v4.4.4  requires  php (^7.1.3)
symfony/process            v4.4.4  requires  php (^7.1.3)
symfony/service-contracts  v2.0.1  requires  php (^7.2.5)
```

…on-php-8

Allow installation on PHP 8

Development for PHPCS 4.x has started in a new `4.0` branch.

This PR is two-fold:

1. On the one hand, it adds testing against PHPCS 4.x to the Travis matrix as an early warning system in case any cross-version compatibility fixes would be needed for the plugin itself.
    The build against `4.x-dev` has been added to `allow_failures` for now.
    Note: as PHPCompatibility currently doesn't allow for PHPCS 4.x (yet), I'm aliasing the install to `3.9.99` for testing purposes in the Travis build only.
2. On the other hand, it allows installation in combination with PHPCS 4.x, which will in turn allow external standards which `require` this plugin, to start testing their standard against the PHPCS 4.x-dev branch.
    As it was now, external standards would get the plugin at version `0.4.4` as that's the last version with a PHPCS `*` constraint.

Fix case of class name

…s-4.x

Test with PHPCS 4.x and allow installation when using PHPCS 4.x

Minimal changes to updated the plugin to support Composer 2.0 which is expected late May/beginning of June.

Changes are based on guidance found in:
* #108
* composer/composer#8726
* https://github.com/composer/composer/blob/master/UPGRADE-2.0.md#for-integrators-and-plugin-authors

Tested by danepowell and myself (Windows 7). Further testing would be very welcome!

Fixes 108

Support Composer 2.0