TEST Add nightly (aka PHP 8) to build matrix

.travis.yml

@@ -49,6 +49,16 @@ matrix:
           packages:
             - libonig-dev
 
+    - php: nightly
+      env:
+        - DB=MYSQL
+        - PHPUNIT_TEST=framework
+        - COMPOSER_INSTALL_ARG="--ignore-platform-reqs"
+      addons:
+        apt:
+          packages:
+            - libonig-dev
+
     - php: 7.3
       if: type IN (cron)
       env:
@@ -78,7 +88,7 @@ before_script:
   - composer require silverstripe/recipe-testing:^1 silverstripe/recipe-core:4.x-dev silverstripe/admin:1.x-dev silverstripe/versioned:1.x-dev --no-update
   - if [[ $PHPUNIT_TEST == cms ]]; then composer require silverstripe/recipe-cms:4.x-dev --no-update; fi
   - if [[ $PHPCS_TEST ]]; then composer global require squizlabs/php_codesniffer:^3 --prefer-dist --no-interaction --no-progress --no-suggest -o; fi
-  - composer install --prefer-source --no-interaction --no-progress --no-suggest --optimize-autoloader --verbose --profile
+  - composer install --prefer-source --no-interaction --no-progress --no-suggest --optimize-autoloader --verbose --profile $COMPOSER_INSTALL_ARG
 
 # Log constants to CI for debugging purposes
   - php ./tests/dump_constants.php