Update .travis.yml

Author: orkhanahmadov

.travis.yml

@@ -31,6 +31,8 @@ matrix:
           env: LARAVEL_VERSION=~7.0  TESTBENCH_VERSION=~5.0  PHPUNIT_VERSION=~9.0
         - php: 7.4
           env: LARAVEL_VERSION=~8.0  TESTBENCH_VERSION=~6.0  PHPUNIT_VERSION=~9.0
+        - php: 8.0
+          env: LARAVEL_VERSION=~8.0  TESTBENCH_VERSION=~6.0  PHPUNIT_VERSION=~9.0
 
 before_install:
     - travis_retry composer self-update