PHP 8 support (#542)

* PHP 8 support, test workflow syntax

* Try using a custom data structure in the workflow

* Try using matrix include

* Default PHP target

* single include

* fix composer package name

* [WIP] Try using dev-master for valuestore

* Update spatie/valuestore constraint to be stable

.github/workflows/ci.yml

@@ -15,16 +15,18 @@ jobs:
 
     strategy:
       matrix:
-        laravel: [
-          "^6.0",
-          "^7.0",
-          "^8.0"
-        ]
-    
+        php_version: ["7.4"]
+        php_target: ["php:7.4-cli"]
+        laravel: ["^6.0", "^8.0"]
+        include:
+          - php_version: "8.0"
+            php_target: "php:8.0.0RC5-cli"
+            laravel: "^8.0"
+
     steps:
     - uses: actions/checkout@v2
     - name: Start docker containers
-      run: docker-compose up -d
+      run: PHP_TARGET=${{ matrix.php_target }} PHP_VERSION=${{ matrix.php_version }} docker-compose up -d
     - name: Install dependencies
       run: docker-compose exec -T test composer require --no-interaction "laravel/framework:${{ matrix.laravel }}"
     - name: Run tests