Enable PHP8 install (#30)

Author: dkarlovi

.travis.yml

@@ -1,5 +1,6 @@
 language: php
 php:
   - 7.4
+  - 8.0
 install: composer install
 script: vendor/bin/phpunit

composer.json

@@ -22,7 +22,7 @@
         }
     },
     "require": {
-        "php": "^7.4",
+        "php": "^7.4|^8.0",
         "ext-dom": "*",
         "symfony/dom-crawler": "^3.0|^4.0|^5.0",
         "symfony/css-selector": "^3.0|^4.0|^5.0",