Merge pull request #99 from xeno010/guzzle7

guzzle ^7.0.0 support

Author: mariacha

composer.json

@@ -7,7 +7,7 @@
   "homepage": "https://github.com/thinkshout/mailchimp-api-php",
   "require": {
     "php": ">=5.4.0",
-    "guzzlehttp/guzzle": "^6.2.1"
+    "guzzlehttp/guzzle": "^6.2.1|^7.0.0"
   },
   "require-dev": {
     "phpunit/phpunit": "^6.2.2"