Update PHP dependency up to 8.0

Author: Romain Follet

.travis.yml

@@ -12,6 +12,8 @@ matrix:
         - php: 7.1
         - php: 7.2
         - php: 7.3
+        - php: 7.4
+        - php: 8.0
 
 cache:
   directories:

composer.json

@@ -16,7 +16,7 @@
         "test": "vendor/bin/phpunit"
     },
     "require": {
-        "php": "^5.6|^7.0",
+        "php": "^5.6|^7.0|^8.0",
         "symfony/framework-bundle": "~3.0|~4.0|~5.0",
         "florianv/swap": "^4.0"
     },