Allow guzzle 6 (#4)

* Set minimum php version to 7.2 and minimum Guzzle version to 7.0.1

* Allow guzzle 6

Author: DanielGSoftware

composer.json

@@ -3,7 +3,7 @@
     "description": "Official supported client for Postbode.nu (https://postbode.nu)",
     "require": {
         "php": "~7.0",
-        "guzzlehttp/guzzle": "^7.0.1"
+        "guzzlehttp/guzzle": "~6.0|^7.0.1"
     },
     "license": "MIT",
     "authors": [{