Run CI against PHP 8.0

.travis.yml

@@ -3,6 +3,8 @@ dist: bionic
 matrix:
   include:
     # jobs for each supported php version
+    - language: php
+      php: nightly # PHP 8.0
     - language: php
       php: 7.4
     - language: php
@@ -43,6 +45,7 @@ cache:
 
 install:
   # install/update composer and php dependencies
+  - if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then export TRAVIS_PHP_VERSION="8.0.0"; fi
   - composer config --unset platform && composer config platform.php $TRAVIS_PHP_VERSION
   - composer update
 

composer.json

@@ -28,7 +28,7 @@
     "require-dev": {
         "roave/security-advisories": "dev-master",
         "phpunit/phpcov": "*",
-        "phpunit/phpunit": "^7.5 || ^8.0",
+        "phpunit/phpunit": "^7.5 || ^8.0 || ^9.0",
         "squizlabs/php_codesniffer": "3.*"
     },
     "suggest": {