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

Author: nicolas-grekas

composer.json

@@ -27,10 +27,10 @@
         "symfony/polyfill-mbstring": "~1.0",
         "symfony/filesystem": "^3.4|^4.0|^5.0",
         "symfony/finder": "^3.4|^4.0|^5.0",
-        "symfony/routing": "^4.4|^5.0"
+        "symfony/routing": "^4.4.12|^5.1.4"
     },
     "require-dev": {
-        "doctrine/annotations": "~1.7",
+        "doctrine/annotations": "^1.10.4",
         "doctrine/cache": "~1.0",
         "paragonie/sodium_compat": "^1.8",
         "symfony/asset": "^3.4|^4.0|^5.0",
@@ -60,13 +60,13 @@
         "symfony/yaml": "^3.4|^4.0|^5.0",
         "symfony/property-info": "^3.4|^4.0|^5.0",
         "symfony/web-link": "^4.4|^5.0",
-        "phpdocumentor/reflection-docblock": "^3.0|^4.0",
-        "twig/twig": "^1.41|^2.10|^3.0"
+        "phpdocumentor/reflection-docblock": "^3.0|^4.0|^5.0",
+        "twig/twig": "^1.43|^2.13|^3.0.4"
     },
     "conflict": {
         "doctrine/persistence": "<1.3",
-        "phpdocumentor/reflection-docblock": "<3.0",
-        "phpdocumentor/type-resolver": "<0.2.1",
+        "phpdocumentor/reflection-docblock": "<3.0|>=3.2.0,<3.2.2",
+        "phpdocumentor/type-resolver": "<0.3.0|1.3.*",
         "phpunit/phpunit": "<4.8.35|<5.4.3,>=5.0",
         "symfony/asset": "<3.4",
         "symfony/browser-kit": "<4.3",