Upgrade to php8

Author: akondas

.gitignore

@@ -1,3 +1,4 @@
 /vendor/
 .php_cs.cache
 /build
+.phpunit.result.cache

.travis.yml

@@ -10,13 +10,12 @@
         }
     ],
     "require": {
-        "php": "^7.2",
+        "php": "^8.0",
         "react/http": "^0.8.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.0",
-        "symplify/easy-coding-standard": "^3.2",
-        "phpstan/phpstan": "^0.10.3"
+        "phpunit/phpunit": "^9.5",
+        "phpstan/phpstan": "^1.6"
     },
     "autoload": {
         "psr-4": {