See if we can get travis to run unit tests on actual php8

.travis.yml

@@ -19,11 +19,9 @@ jobs:
    - php: 7.2
    - php: 7.3
    - php: 7.4
-   - php: nightly
+   - php: 8.0
      install: travis_retry composer config platform.php 7.4.0 && composer install
   fast_finish: true
-  allow_failures:
-   - php: nightly # PHP 8 is still in beta
 
 services:
   - memcached

composer.json

@@ -40,7 +40,7 @@
         }
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.5 || ^6.5 || ^5.7 || ^4.8",
+        "phpunit/phpunit": "^9.5 || ^8.5 || ^7.5 || ^6.5 || ^5.7 || ^4.8",
         "smarty/smarty-lexer": "^3.1"
     }
 }