drop support of very old php-s

Author: Toma Zoltan

.github/workflows/ci.yml

@@ -19,8 +19,6 @@ jobs:
           - 7.0
           - 5.6
           - 5.5
-          - 5.4
-          - 5.3
     steps:
       - uses: actions/checkout@v2
       - uses: shivammathur/setup-php@v2

composer.json

@@ -17,7 +17,7 @@
         "psr-4": { "LeeShan87\\Tests\\React\\MultiLoop\\": "tests/" }
     },
     "require": {
-        "php": ">=5.3",
+        "php": ">=5.5",
         "react/event-loop": "~1.0"
     },
     "require-dev": {