Travis CI: run composer update instead of install

And ignore PHP 7.1 platform requirement, in order to get matching version of PHPUnit

.travis.yml

@@ -43,7 +43,7 @@ cache:
 
 install:
   # install/update composer and php dependencies
-  - composer install --prefer-dist
+  - composer update --ignore-platform-reqs
 
 before_script:
   - PATH=${PATH//:\.\/node_modules\/\.bin/}

composer.json

@@ -28,7 +28,7 @@
     "require-dev": {
         "roave/security-advisories": "dev-master",
         "phpunit/phpcov": "*",
-        "phpunit/phpunit": "^7.5",
+        "phpunit/phpunit": "^7.5 || ^8.0 || ^9.0",
         "squizlabs/php_codesniffer": "3.*"
     },
     "suggest": {