Remove async package

Author: joelwurtz

composer.json

@@ -16,7 +16,6 @@
         "symfony/options-resolver": "^2.6 || ^3.0 || ^4.0 || ^5.0"
     },
     "require-dev": {
-        "concurrent-php/async-api": "dev-master",
         "friendsofphp/php-cs-fixer": "^2.2",
         "php-http/client-integration-tests": "dev-master",
         "php-http/message": "^1.0",