Updates phpunit requirement.

Author: mvdnbrk

composer.json

@@ -25,7 +25,7 @@
     "require-dev": {
         "larapack/dd": "^1.1",
         "nunomaduro/collision": "^3.0",
-        "phpunit/phpunit": "^8.0"
+        "phpunit/phpunit": "^7.5|^8.0"
     },
     "autoload": {
         "psr-4": {