Use phpunit 8, fix #56

Author: michielbdejong

solid/composer.json

@@ -26,6 +26,6 @@
         "lcobucci/jwt": "^3.3"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9"
+        "phpunit/phpunit": "^8"
     }
 }