Add PHP 8 to the CI builds

Also allow higher versions of PHPUnit as PHPUnit 4.8 does not work on
PHP 8.

Author: oliverklee

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        php-version: [ '5.6', '7.0', '7.1', '7.2', '7.3', '7.4' ]
+        php-version: [ '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0' ]
 
     steps:
       - name: Checkout
@@ -39,7 +39,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-version: [ '5.6', '7.0', '7.1', '7.2', '7.3' ]
+        php-version: [ '5.6', '7.0', '7.1', '7.2', '7.3', '8.0' ]
         coverage: [ 'none' ]
         include:
           - php-version: 7.4

composer.json

@@ -12,7 +12,7 @@
         "php": ">=5.6.20"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8.36",
+        "phpunit/phpunit": "^4.8.36 || ^7.5.20",
         "codacy/coverage": "^1.4"
     },
     "autoload": {