Travis: run the tests on PHP 8/nightly

Now 12 has been merged, let's start running the tests on PHP 8.0 to see what needs fixing.

Author: jrfnl

.travis.yml

@@ -12,14 +12,25 @@ matrix:
     - php: 7.2
     - php: 7.3
     - php: 7.4
+    - php: "nightly"
+
+  allow_failures:
+    # Allow failures for unstable builds.
+    - php: "nightly"
 
 cache:
   directories:
     - vendor
     - $HOME/.composer/cache
 
 install:
-  - composer install --no-interaction
+  - |
+    if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then
+      # Not all dependencies allow for installing on PHP 8 yet, so ignore platform requirements.
+      composer install --no-interaction --ignore-platform-reqs
+    else
+      composer install --no-interaction
+    fi
 
 script:
   - ./vendor/bin/parallel-lint --exclude vendor .