Update to version 3 of core (#244)

* Update to version 3 of core

* [Flysystem] Allow many version of core

* [Symfony] Allow core ^0.1 || ^0.2

Author: Nyholm

composer.json

@@ -10,7 +10,7 @@
     "require": {
         "php": "^7.2.5",
         "ext-json": "*",
-        "async-aws/core": "^0.2",
+        "async-aws/core": "^0.3",
         "symfony/http-client-contracts": "^1.0 || ^2.0"
     },
     "extra": {