Do not support Guzzle 4

Guzzle 4's `Emitter::once()` method (and potentially others) has a
syntax error that is detected in PHP 7.1 and up (an attempt to bind a
variable name by the same name as a parameter to the closure). As such,
we can no longer support it.

Author: weierophinney

composer.json

@@ -22,7 +22,7 @@
     },
     "require-dev": {
         "doctrine/migrations": "^1.0",
-        "guzzlehttp/guzzle": "^4.2.4 || ^5.3.2 || ^6.3.3",
+        "guzzlehttp/guzzle": "^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",