Upgrade phpunit

Author: AydinHassan

.gitignore

@@ -5,3 +5,4 @@ program.php
 program.js
 /solution
 exercises/*/solution/vendor
+.phpunit.result.cache

composer.json

@@ -14,11 +14,11 @@
     "require" : {
         "php" : ">=7.2",
         "ext-pdo_sqlite": "*",
-        "php-school/php-workshop": "^2.2",
+        "php-school/php-workshop": "2.2",
         "hoa/socket": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7",
+        "phpunit/phpunit": "^7.0 | ^8.0",
         "squizlabs/php_codesniffer": "^3.5"
     },
     "autoload" : {