Relax dependency on Guzzle

Resolves #8

Author: robertlemke

composer.json

@@ -16,8 +16,8 @@
     },
     "require": {
         "neos/flow": "^4.0 || ^5.0 || ^6.0 || dev-master",
-        "guzzlehttp/guzzle": "6.3.*",
-        "league/oauth2-client": "2.*",
+        "guzzlehttp/guzzle": "^6.3",
+        "league/oauth2-client": "^2.0",
         "ramsey/uuid": "^3.0 || ^4.0",
         "paragonie/sodium_compat": "^1.10"
     },