[travis] use PHP 8.0 to patch return types and run deps=low

Author: nicolas-grekas

composer.json

@@ -28,7 +28,7 @@
         "symfony/service-contracts": "^1.0|^2"
     },
     "require-dev": {
-        "guzzlehttp/promises": "^1.3.1",
+        "guzzlehttp/promises": "^1.4",
         "nyholm/psr7": "^1.0",
         "php-http/httplug": "^1.0|^2.0",
         "psr/http-client": "^1.0",