Allow HTTPlug 2 (and hence PSR-18)

Author: Jean85

composer.json

@@ -14,7 +14,7 @@
         "php": "^5.5 || ^7.0"
     },
     "require-dev": {
-        "php-http/httplug": "^1.0",
+        "php-http/httplug": "^1.0|^2.0",
         "php-http/message-factory": "^1.0",
         "puli/composer-plugin": "1.0.0-beta10",
         "phpspec/phpspec": "^2.4",