Depend on nyholm/psr7:^1.0 (#15)

* Depend on nyholm/psr7:^1.0

* Support both, PHP 7.0 and 7.1

Author: claudiu-cristea

composer.json

@@ -21,7 +21,7 @@
         "phpunit/phpunit": "^6.1",
         "psr/http-message-implementation": "^1.0",
         "php-http/mock-client": "^1.0",
-        "nyholm/psr7": "^0.2.2"
+        "nyholm/psr7": "^0.2.2|^1.0.0"
     },
     "require-dev": {
         "php-http/curl-client": "^1.7",