Bumped guzzlehttp/psr7 to v2.0 + minimal php version to 7.2 accordingly (#23)

Author: evertharmeling

composer.json

@@ -16,9 +16,9 @@
         }
     ],
     "require": {
-        "php": "^7.1",
+        "php": "^7.2",
         "ext-json": "*",
-        "guzzlehttp/psr7": "^1.3",
+        "guzzlehttp/psr7": "^2.0",
         "psr/http-client": "^1.0"
     },
     "require-dev": {

tests/FH/PostcodeAPI/ClientTest.php

@@ -5,14 +5,13 @@
 use FH\PostcodeAPI\Client;
 use FH\PostcodeAPI\Exception\InvalidApiKeyException;
 use FH\PostcodeAPI\Exception\ServerErrorException;
+use GuzzleHttp\Psr7\Message;
 use GuzzleHttp\Psr7\Response;
 use Http\Mock\Client as MockClient;
 use PHPUnit\Framework\Assert;
 use PHPUnit\Framework\TestCase;
 use stdClass;
 
-use function GuzzleHttp\Psr7\parse_response;
-
 /**
  * @author Gijs Nieuwenhuis <gijs.nieuwenhuis@freshheads.com>
  */
@@ -99,7 +98,7 @@ public function testClientThrowsExceptionWhenInvalidInputIsSupplied(): void
 
     private function loadMockResponse(string $name): Response
     {
-        return parse_response(file_get_contents(__DIR__ . "/../../Mock/{$name}"));
+        return Message::parseResponse(file_get_contents(__DIR__ . "/../../Mock/{$name}"));
     }
 
     private function applyIsFreshheadsAddressAssertions(stdClass $address): void