Prepare for psr/http-client (#274)

Author: Nyholm

composer.json

@@ -21,7 +21,7 @@
         "php-http/client-implementation": "^1.0",
         "php-http/cache-plugin": "^1.4",
         "php-http/discovery": "^1.0",
-        "php-http/httplug": "^1.0",
+        "php-http/httplug": "^1.0 || ^2.0",
         "php-http/logger-plugin": "^1.0",
         "php-http/message": "^1.4",
         "php-http/message-factory": "^1.0.2",