Add support for Guzzle 7 (#3)

* Set minimum php version to 7.2 and minimum Guzzle version to 7.0.1
* Make it non-breaking for people who use Guzzle 6
* Allow Guzzle 7.x versions instead of 7.0.x
* Allow Guzzle 7.x versions instead of 7.0.x

Author: DanielGSoftware

composer.json

@@ -2,8 +2,8 @@
     "name": "postbode/postbode-api",
     "description": "Official supported client for Postbode.nu (https://postbode.nu)",
     "require": {
-        "php": ">=7.0",
-        "guzzlehttp/guzzle": "~6.0"
+        "php": "~7.0",
+        "guzzlehttp/guzzle": "^7.0.1"
     },
     "license": "MIT",
     "authors": [{