Remove async package dependency

Author: sagikazarmark

composer.json

@@ -15,7 +15,6 @@
         "phpunit/phpunit": "^4.4",
         "php-http/discovery": "~0.2@dev",
         "php-http/httplug": "~1.0@dev",
-        "php-http/httplug-async": "~0.1@dev",
         "guzzlehttp/psr7": "^1.0",
         "th3n3rd/cartesian-product": "^0.3"
     },