Allow newer php-http/discovery versions

Author: ddeboer

composer.json

@@ -14,7 +14,7 @@
         "php": ">=5.4",
         "php-http/httplug": "^1.0@beta",
         "php-http/client-common": "~0.1",
-        "php-http/discovery": "^0.5"
+        "php-http/discovery": "~0.5"
     },
     "require-dev": {
         "phpspec/phpspec": "^2.4",