add php7.4 to travis

Author: sergiy-petrov

.travis.yml

@@ -23,6 +23,10 @@ matrix:
           env: LARAVEL='6.*' TESTBENCH='4.*' COMPOSER_FLAGS='--prefer-lowest'
         - php: 7.3
           env: LARAVEL='6.*' TESTBENCH='4.*' COMPOSER_FLAGS='--prefer-stable'
+        - php: 7.4
+          env: LARAVEL='6.*' TESTBENCH='4.*' COMPOSER_FLAGS='--prefer-lowest'
+        - php: 7.4
+          env: LARAVEL='6.*' TESTBENCH='4.*' COMPOSER_FLAGS='--prefer-stable'
 
 before_install:
     - travis_retry composer self-update