Travis CI: Add PHP 8.0 target

.travis.yml

@@ -16,6 +16,7 @@ jobs:
     - php: 7.2
     - php: 7.3
     - php: 7.4
+    - php: 8.0
     - php: nightly
     - php: hhvm-3.27 # until Sep 2019
     - php: hhvm-3.30 # until Nov 2019