Update composer.json

Added support for HTTPlug 2.0

Author: grtumanyan

composer.json

@@ -14,7 +14,7 @@
   },
   "require": {
     "php": "^5.6 || ^7.0",
-    "php-http/httplug": "^1.0",
+    "php-http/httplug": "^1.0 || ^2.0",
     "php-http/message": "^1.0",
     "php-http/client-implementation": "^1.0",
     "php-http/discovery": "^1.0"