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

Author: nicolas-grekas

composer.json

@@ -43,7 +43,7 @@
         "symfony/twig-bundle": "^4.4|^5.0",
         "symfony/validator": "^3.4|^4.0|^5.0",
         "symfony/yaml": "^3.4|^4.0|^5.0",
-        "twig/twig": "^1.41|^2.10|^3.0"
+        "twig/twig": "^1.43|^2.13|^3.0.4"
     },
     "conflict": {
         "symfony/browser-kit": "<4.2",