Updating min phpunit 8 version to prevent error with PHP 7.2 + Testing PHP 8

Author: gpressutto5

.travis.yml

@@ -5,6 +5,8 @@ php:
   - 7.2
   - 7.3
   - 7.4
+  - 8.0
+  - 8.1
 
 sudo: false
 

composer.json

@@ -16,13 +16,13 @@
     "require": {
         "php": ">=7.1.3",
         "guzzlehttp/guzzle": "^6.3 || ^7.0",
-        "illuminate/notifications": "^5.8 || ^6.0 || ^7.0 || ^8.0|^9.0",
-        "illuminate/support": "^5.8 || ^6.0 || ^7.0 || ^8.0|^9.0",
+        "illuminate/notifications": "^5.8 || ^6.0 || ^7.0 || ^8.0 || ^9.0",
+        "illuminate/support": "^5.8 || ^6.0 || ^7.0 || ^8.0 || ^9.0",
         "laravel/slack-notification-channel": "^2.0"
     },
     "require-dev": {
         "orchestra/testbench": "*",
-        "phpunit/phpunit": "^7.0 || ^8.0 || ^9.0"
+        "phpunit/phpunit": "^7.0 || ^8.5.28 || ^9.0"
     },
     "autoload": {
         "psr-4": {