Add missing php-http/promise interface

Author: firstred

composer.json

@@ -19,6 +19,7 @@
         "ext-json": "*",
         "composer/ca-bundle": "^1.2",
         "guzzlehttp/psr7": "^1.7 || ^2.0",
+        "php-http/promise": "^1.0",
         "psr/cache": "^1.0 || ^2.0 || ^3.0",
         "psr/http-message": "^1.0",
         "psr/log": "^1.0",