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

Author: nicolas-grekas

composer.json

@@ -24,7 +24,7 @@
         "symfony/yaml": "^3.4|^4.0|^5.0",
         "symfony/expression-language": "^3.4|^4.0|^5.0",
         "symfony/dependency-injection": "^3.4|^4.0|^5.0",
-        "doctrine/annotations": "~1.2",
+        "doctrine/annotations": "^1.10.4",
         "psr/log": "~1.0"
     },
     "conflict": {