updated composer file

Author: Ken Seah

composer.json

@@ -7,7 +7,7 @@
         "ext-curl": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "4.0.*",
+        "phpunit/phpunit": "6.2.*",
         "vlucas/phpdotenv": "^2.0"
     },
     "autoload": {
@@ -16,7 +16,7 @@
         }
     },
     "homepage": "https://github.com/drewm/mailchimp-api",
-    "authors": [ 
+    "authors": [
         {
             "name": "Drew McLellan",
             "email": "drew.mclellan@gmail.com",