Travis: add build against PHP 8.0

PHP 8.0 has been branched off two months ago, so `nightly` is now PHP 8.1 and in the mean time PHP 8.0 was released last week.

As of today, there is a PHP 8.0 image available on Travis.

This PR adds two new builds against PHP 8.0 to the matrix and, as PHP 8.0 has been released, these builds are not allowed to fail.

Author: jrfnl

.travis.yml

@@ -100,6 +100,11 @@ jobs:
     # PHPCS is only compatible with PHP 7.4 as of version 3.5.0.
     - php: 7.4
       env: PHPCS_VERSION="3.5.0"
+    - php: 8.0
+      env: PHPCS_VERSION="dev-master" LINT=1
+    # PHPCS is only compatible with PHP 8.0 as of version 3.5.7.
+    - php: 8.0
+      env: PHPCS_VERSION="3.5.7"
 
     # PHP 5.4/5.5 need trusty.
     - php: 5.5
@@ -146,8 +151,8 @@ before_install:
   # --prefer-dist will allow for optimal use of the travis caching ability.
   # The Composer PHPCS plugin takes care of setting the installed_paths for PHPCS.
   - |
-    if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then
-      # Not all dependencies allow for installing on nightly yet, so ignore platform requirements.
+    if [[ $TRAVIS_PHP_VERSION == "nightly" || $TRAVIS_PHP_VERSION == "8.0" ]]; then
+      # Not all dependencies allow for installing on PHP 8.0+, so ignore platform requirements.
       travis_retry composer install --prefer-dist --no-suggest --ignore-platform-reqs
     else
       travis_retry composer install --prefer-dist --no-suggest