Allow guzzlehttp/psr7 ^1.7|^2.1 (#66)

Author: rtuin

composer.json

@@ -16,7 +16,7 @@
     "ext-json": "*",
     "guzzlehttp/guzzle": "^7.0",
     "guzzlehttp/promises": "^1.4.0",
-    "guzzlehttp/psr7": "^1.7",
+    "guzzlehttp/psr7": "^1.7|^2.1",
     "beberlei/assert": "~2.7|~3.0",
     "flix-tech/avro-php": "^4.1"
   },