Use Guzzle 4, 5, and 6 only

Guzzle 3 has been EOL for 5 years. 4 has been EOL for 3 years (though it
had a release ~21 months ago). Guzzle 5 is still being maintained, but
is considered legacy to the PSR-7-based Guzzle 6. Versions 4 and 5 have
the same API for our purposes, so we can support those two reasonably,
and add support for version 6 (which, while we had code to allow it, it
never was actually tested, and, in fact, did not work without
deprecation notices).

Author: weierophinney

composer.json

@@ -22,8 +22,7 @@
     },
     "require-dev": {
         "doctrine/migrations": "^1.0",
-        "guzzle/http": "^3.0",
-        "guzzle/plugin-mock": "^3.0",
+        "guzzlehttp/guzzle": "^4.2.4 || ^5.3.2 || ^6.3.3",
         "mikey179/vfsStream": "^1.6",
         "php-amqplib/php-amqplib": "^2.0",
         "phpunit/phpunit": "^5.7.27 || 6.5.8 || ^7.1.2",
@@ -37,7 +36,7 @@
         "ext-bcmath": "Required by Check\\CpuPerformance",
         "sensiolabs/security-checker": "Required by Check\\SecurityAdvisory",
         "symfony/yaml": "Required by Check\\YamlFile",
-        "guzzle/http": "Required by Check\\GuzzleHttpService",
+        "guzzlehttp/guzzle": "Required by Check\\GuzzleHttpService",
         "predis/predis": "Required by Check\\Redis",
         "videlalvaro/php-amqplib": "Required by Check\\RabbitMQ",
         "doctrine/migrations": "Required by Check\\DoctrineMigration"